Need advice for ADC Buffer

[BrianHG]

poorly taped battery contacts as well...

[bson]

Even though the inputs are differential they need to be referenced to a common ground.  Consider if one is 1V and the other 0.5V, for a -0.5V differential voltage, how could you ever have a return current?  KCL will tell you if current goes in it needs to come out...  For SOME sources it will work to differentially terminate them, but that requires the drivers to be able to sink.  And even then you risk having them floating too close to the supplies (or outside).

[Inverted18650]

I build this unit and have been pleased with its performance. Mr. Louis is quite thorough as well.

[David Hess]

Even though the inputs are differential they need to be referenced to a common ground.

Take a close look at the schematic.  The two operational amplifiers are used in parallel to reduce noise and not configured as a differential amplifier.  The measurement is singled ended to common so there is no need for an input bias current return although one is included anyway.

[Kleinstein]

Even with AZ OPs it does not work with direct parallel connection of the OPs outputs. Both OPs should have an individual 100 Ohms or similar resistor towards the ADC.

There quite a few AZ OPs to choose from, so paralleling them is a bit unusual, except for the very low noise end. Something like an AD8551 might about replace the 2 LTC2051 amps.

[Crossphased]

I build this unit and have been pleased with its performance. Mr. Louis is quite thorough as well.

Hi Inverted, thank you for the video! Going to check it out tonight.

[Crossphased]

Even with AZ OPs it does not work with direct parallel connection of the OPs outputs. Both OPs should have an individual 100 Ohms or similar resistor towards the ADC.

There quite a few AZ OPs to choose from, so paralleling them is a bit unusual, except for the very low noise end. Something like an AD8551 might about replace the 2 LTC2051 amps.

Thanks Kleinstein,
Yes David kindly pointed that out to me as well. definitely will do that in the final implementation

[Crossphased]

As a bit of an update,

there seems to be a little bit of non-linearity for the In vs. Out voltage to the paralleled op amps. For instance at an input voltage of less than 1 volt, there seems to be a larger discrepancy between the Input and Output voltage, and a smaller discrepancy when the input is above 4 volts. However the source of the input I used wasn't exactly a "stable" voltage.  The source was an old DC power supply: AC transformer to rectifier to filter caps. A good amount of ripple at 120 cycles. Would this quasi AC voltage have an effect on the accuracy?

For instance for input voltages < 1 V there was ~ .000016 V difference between In and Out of op amps
Input range of 1 -3 V there was around .000010 - .000015 V between In and Out
Input of 4-5 V produced a discrepancy of .000007 V between In and Out

I'm wondering if this is because of an error with my circuit, or is something that is expected. I just don't know so I'm asking you guys! I'm going to take more readings tonight with stable DC voltages and I'll come back with more refined data. Thanks as always

[Crossphased]

poorly taped battery contacts as well...

Hi Brian,

Is there a preferred method for battery contacts if you don't have a battery holder on hand? What I did was take some copper braid you use for de-soldering, soldered on leads, then put the copper braid across the battery terminals, and taped tightly. Is there an alternative method you like?

[Crossphased]

Even with AZ OPs it does not work with direct parallel connection of the OPs outputs. Both OPs should have an individual 100 Ohms or similar resistor towards the ADC.

There quite a few AZ OPs to choose from, so paralleling them is a bit unusual, except for the very low noise end. Something like an AD8551 might about replace the 2 LTC2051 amps.

I just looked at the specs for the 8551, Holy Cow! Much better than the 2051. I chose the 2051 because the example circuit in the 2440 used the 2051. I'm new to op amps, and am not aware of all the part offerings. Next time I'll definitely ask here first

[Kleinstein]

The AD8551 is lower noise than the LTC2051, but it also has a little higher bias current. There is a kind of trade-off between these two the very low bias AZ OPs (e.g. LTC2050, max4238) have a higher noise and the low noise ones (e.g. ADA4522, LTC2057, MCP6V91, OPA180) have a higher bias current. There are also types in between.

It might be a good idea to have a kind of filtering at the input, to suppress the high frequency current spikes coming from the AZ OPs. The currently shown RC combination does not look good in the respect.

[Crossphased]

Hello Friends,

I return with more good news! I inserted the 100 Ohm resistors you recommended into the parallel outputs. This improved measurements considerably.

Here is the schematic:


The power supply voltages were set at +6.5V, and -1.55V

For these measurements the delta between input and output voltage was measured. In this round of data the output was always negative with respect to the input, it was slightly below the input. That is the convention for this data. The input was slowly varied using a 9V battery and a pot. A measurement was taken if the Vdiff changed.

Vin(Volts) Vdiff (uV)
0 .... 13
.22 .... 12
.28 .... 11
.50 .... 10
.60 .... 9
.88 .... 8
1.00 .... 7
1.13 .... 6
1.39 .... 5
1.62 .... 4
1.80 .... 3
2.61 .... 4
2.95 .... 4
3.27 .... 2
4.86 ... 3
5.04 .... 4
5.40 .... 5
5.70 .... 7
5.79 .... 12

 Looks much better! Max error of 12 uV. It seems like the best accuracy is around the middle of the power supply range. From your guys experience, is this typical? Or is the error typically uniform across the range? Is this the kind of real world numbers one would normally see?

What do you guys think the reason is for the 13 uV offset at 0? The datasheet lists a max of 5 uV offset.

[Crossphased]

The AD8551 is lower noise than the LTC2051, but it also has a little higher bias current. There is a kind of trade-off between these two the very low bias AZ OPs (e.g. LTC2050, max4238) have a higher noise and the low noise ones (e.g. ADA4522, LTC2057, MCP6V91, OPA180) have a higher bias current. There are also types in between.

It might be a good idea to have a kind of filtering at the input, to suppress the high frequency current spikes coming from the AZ OPs. The currently shown RC combination does not look good in the respect.

Hi Kleinstein, thanks for the info!
 What would you suggest on the input? RC filter? LC? what ballpark component values?

Also, you're saying op amps with a larger bias current have lower noise... are there any downsides to a larger bias current?

[Kleinstein]

Off cause there are downside of more current at the input. Together with resistance needed for protection or from the source this adds a small voltage. Usually more bias current also comes with more current noise, though the RMS value is still rather low for the AZ amps.

If externally accessible the input would need some resistance for protection. Something like the 100 K shown below could be about the right order of magnitude. The resistors should be made to also sustain a significant voltage (e.g. 300 or 500 V). So this could require 2 or 3 resistors in series. For the very high frequencies (e.g. cell phone) an extra inductance / ferrite bead is likely a good idea too. The capacitance to ground should not be so large, to allow a reasonable fast response and limit transient input currents. So something like 2 times 1 nF (at the OP's input and at the very input) might be a first start.

The series resistance already adds some noise (e.g. about 50 nV/SQRT(Hz) and thus about as much as the AD8551 has). So there is no need to go for a super low noise amplifier.

For additional protection is might be good do have clamping diodes - if very low bias in the pA range is aimed for, this would be something like a pair of back to back diodes (e.g. BAV199) towards 2 zeners with bootstrapping from the output. The Bootstrapping part could also be used in Filtering.

[David Hess]

Looks much better! Max error of 12 uV. It seems like the best accuracy is around the middle of the power supply range. From your guys experience, is this typical? Or is the error typically uniform across the range? Is this the kind of real world numbers one would normally see?

What do you guys think the reason is for the 13 uV offset at 0? The datasheet lists a max of 5 uV offset.

The 120dB common mode rejection ratio could explain 1uV/V of offset change which is not enough.

The change in input bias current over common mode range according to the datasheet could amount to almost 100pA which would produce an input offset change of 10uV through the 100k resistor so that may cover it.  This could be tested by temporarily shorting the input resistors and running the test again.

Depending on how much input protection is required, the value of the input resistors could be lowered.  Then some of the input protection could be made up for by adding low leakage shunt diodes.  Another thing I might try is a pair of back to back depletion mode MOSFETs with a relatively low value resistor between them to replace the existing resistor.  (1) At low currents, the MOSFETs are fully on and the total resistance is low.  At high currents, the MOSFETs shut off limiting the current into the operational amplifier's inputs.  A more complex design might bootstrap the input amplifier to remove all common mode related errors but that is not going to work without a larger supply voltage.

High input impedance voltmeters do not commonly use chopper stabilized amplifiers by themselves for their input stage.  It would be interesting to see how a bipolar LT1012 or precision JFET part compares when its input offset is nulled.  There may be better chopper stabilized parts than the LTC2051.

(1) Figure 4 on PDF page 2 of Supertex/Microchip application note 66.

[Kleinstein]

There are some  DMMs that use chopper-stabilzed OPs at the input. E.g. the Keiththly 2000 maybe K2002 and many the Prema DMMs. The Datron 1281/1271 also use a copper amplifier, though made with discrete JFETs.

There are also old designs with JFET OPs or low bias BJT based amps at the input, without extra automatic zero circuitry (e.g. fluke 8050). But these are usually older designs before auto zero became a quasi standard.

[Crossphased]

Hi guys,

I spent most of the weekend writing code to interface with the 2440 over SPI. Things are starting to come together.

I've taken to heart your recommendations for an alternative amplifier than the LTC2051. I read the datasheet for the AD8551. It looks like an excellent part. One thing that concerned me is p19 in the datasheet, Capacitive Load Drive. It stated the 8551 is only capable of driving up to 10 nF. I'm not sure what all the constraints are surrounding this figure, but I'm wondering if this would be a problem with my 1 uF caps on the output?

I also looked at the part AD8628. What is your opinion of that amp in this application? I'm basically looking for your guys' best recommendation.

Another question I have is concerning the choice of passive components. So far I have been using components with leads on them. Do larger, leaded components tend to have more or less noise than their smd counterparts? I can imagine the larger components may have less of a temperature associated noise, but may also capture more parasitic noise. Or maybe the difference in magnitudes between smd and leaded is hardly noticeable. Im curious what your experience has been.

Also one last question regarding input protection-
In the 2440 datasheet it suggests max over and under voltages of .3 V.  I looked at the BAV199 and it has a forward drop of 1V. How do does the diode protect the ADC at that point? Do you connect it to a rail lower than Vcc? Or do the protection diodes on the ADC take up that .7 V?

As always, thank you all for your help!

[Crossphased]

High input impedance voltmeters do not commonly use chopper stabilized amplifiers by themselves for their input stage.  It would be interesting to see how a bipolar LT1012 or precision JFET part compares when its input offset is nulled.  There may be better chopper stabilized parts than the LTC2051.

Hi David,
Thanks for the advice. Would you recommend moving forward with a bipolar amp, or a chopper amp to null offset of a bipolar amp? Or just stick with what I've got? I'm open to whatever you guys would consider best practice for a low noise application. In part, I'm building a functional device, but I always like learning best practices in the process.

[BrianHG]

poorly taped battery contacts as well...

Hi Brian,

Is there a preferred method for battery contacts if you don't have a battery holder on hand? What I did was take some copper braid you use for de-soldering, soldered on leads, then put the copper braid across the battery terminals, and taped tightly. Is there an alternative method you like?

This is tough, on more than 1 front.  Even with a real battery holder, rotating the batteries withing one can lead to voltage differentiation at the 5 digit DVM grade measurements like what you are doing.  Even temperature of the batteries will shift at 5 digits while with a 7.5 or 8.5 digit DVM, you can just hook up a battery and watch the voltage drop every few minutes without any load other than the DVM.

Soldering to the batteries will just cause heat damage on each side of the battery accelerating the voltage drop.  If you do it super quick, just getting the solder to properly flux and bind to the battery contact before the heat makes it too far into the batteries case is trick, but you can try.  If it was me and I had absolutely no other choice, and I didn't care about some slight battery damage, I would solder a bit on a curved edge of the batteries contact, minimal, then after cooling, again use the spot to solder a thin wire.  Then, give the battery a few hours to re-acclimate before measuring with above 5 digit precision.

I know D-cells are over kill, but, the size of them, while soldering such a tiny point on the edge of the contact means less internal heat damage than a size AA battery.

[Kleinstein]

The capacitive drive capability are for having the capacitor directly at the output of the OP.  Even with an OP that is specified for unlimited capacitive drive capability this only means the OP won't oscillate if everything else is good, performance with a lot of capacitance directly at the output would still be poor only one the side of just not oscillating.  To really drive the ADC with it's filtering cap (e.g. 1 µF range) one needs the extra resistors between the OP and the cap.

If one goes for an AZ OP or a low bias conventional precision OP (e.g. LT1012 o a precision JFET type) depends on what type of noise you care more about. The AZ OP will be better with drift and very low frequency (e.g. < 0.1 to 5 Hz), the JFET part can be lower bias. The AZ type OPs tend to have extra higher frequency noise. Most of the AZ OPs are for 5 V supply only - for higher voltages a more normal OP might be easier.

The form factor (e.g. SMD to leaded) can make a difference with resistor excess noise. However in this circuit the resistors do not see a significant voltage / current under normal operation and this would no produce excess noise, but just the normal Johnson noise, that does not depend on the type of resistor but just the resistance. For the protection resistors at the input the larger form might be needed, so they could withstand a high voltage, e.g. from ESD. For this reason it might need a few resistors in series.

The protection diodes would be before the OP, not so much before the ADC.  It depends on the circuit how much voltage actually reaches the OP. It might be needed to have an additional resistance from the point where the diodes clamp the voltage to the OPs input. This way it would be something like half a volt too much, but with something like an additional 10 K in series to limit the current through OP internal diodes to a safe level.  In some cases clamping is also towards a lower voltage - it depends on the type of circuit and the supply range.

[Marco]

For an autoranging meter, protection needs to be at the multiplexer.

[David Hess]

I've taken to heart your recommendations for an alternative amplifier than the LTC2051. I read the datasheet for the AD8551. It looks like an excellent part. One thing that concerned me is p19 in the datasheet, Capacitive Load Drive. It stated the 8551 is only capable of driving up to 10 nF. I'm not sure what all the constraints are surrounding this figure, but I'm wondering if this would be a problem with my 1uF caps on the output?

In this case, they want the output capacitance to ground to lower the high frequency impedance that the ADC sees which could otherwise perturb the amplifier.  Your circuit isolates the output capacitance from the operational amplifier with a resistor and feedback so there is no problem.

Another question I have is concerning the choice of passive components. So far I have been using components with leads on them. Do larger, leaded components tend to have more or less noise than their smd counterparts? I can imagine the larger components may have less of a temperature associated noise, but may also capture more parasitic noise. Or maybe the difference in magnitudes between smd and leaded is hardly noticeable. Im curious what your experience has been.

Leaded components might pick up more noise but the leads provide strain relief.  These issues can be handled in other ways like shielding and cutouts in the printed circuit board for strain isolation.

In the 2440 datasheet it suggests max over and under voltages of .3 V.  I looked at the BAV199 and it has a forward drop of 1V. How do does the diode protect the ADC at that point? Do you connect it to a rail lower than Vcc? Or do the protection diodes on the ADC take up that .7 V?

There are two ways and I might use both.  The diodes can be used in pairs, or more likely a diode paired with a bipolar transistor, so that the clamp voltage is actually one Vbe within the power supply voltages.  A lower value series resistor can be placed between the diode and input so the integrated input protection diodes only see the difference in voltage which will be 10s of millivolts.
 

High input impedance voltmeters do not commonly use chopper stabilized amplifiers by themselves for their input stage.  It would be interesting to see how a bipolar LT1012 or precision JFET part compares when its input offset is nulled.  There may be better chopper stabilized parts than the LTC2051.

Thanks for the advice. Would you recommend moving forward with a bipolar amp, or a chopper amp to null offset of a bipolar amp? Or just stick with what I've got? I'm open to whatever you guys would consider best practice for a low noise application. In part, I'm building a functional device, but I always like learning best practices in the process.

I would socket the operational amplifiers if you can and use what you have now.  You can substitute other parts later for comparison.  If you are using longer integration times, then broadband noise from the operational amplifier is less important and low 1/f noise is more important.

I have used a chopper stabilized amplifier to offset null low noise bipolar parts like the LT1028 and low noise JFET parts which have a relatively high input capacitance and leakage but not a part like the LT1012 so I am not sure how much benefit there would be for it.  It certainly works for JFET parts that have a similar input bias current to the LT1012.

Knowing what I know now, in a low noise high impedance DC application I would cascode the input amplifier to further remove bias current related errors before using a chopper stabilized amplifier to offset null another amplifier.

Soldering to the batteries will just cause heat damage on each side of the battery accelerating the voltage drop.  If you do it super quick, just getting the solder to properly flux and bind to the battery contact before the heat makes it too far into the batteries case is trick, but you can try.  If it was me and I had absolutely no other choice, and I didn't care about some slight battery damage, I would solder a bit on a curved edge of the batteries contact, minimal, then after cooling, again use the spot to solder a thin wire.

Grinding the surface to be soldered down with an emery wheel and using a bit of HCl or acid flux makes the soldering go very quickly to minimize heat damage.  Better is to use a spot welder like they use to attach battery tabs.

[Crossphased]

Hi,

Thank you very much for the very helpful information you have provide me! I appreciate it, it is very kind of you all!

As an update, I've placed an order with digikey for the 8551 operational amplifiers. When they arrive I'm going to build a test circuit like I did with the 2051's and compare performance before doing a final installation. I

In the mean time, I have been working on taking readings from the 2440 over SPI. The 2440 has 5V logic levels, and the MCU I'm using is an arduino due, which has 3.3 logic levels. To perform the level shift and isolation I'm using Si8663. Its like an optocoupler but uses RF instead of light. I had one on hand so I used it. I've been getting some very odd readings out of the ADC, and I'm not sure if its from an error reading the SPI bus, or an error in my circuit.  Right now there are no op amps in front of the ADC, I'm interfacing directly to the ADC inputs.

Below is some readings I have been getting.from the ADC: (Vref = 5.0 V, +In is connected to -In through 10K resistor)
With inputs shorted: ADC reads 2.49997 V
with input connected to VREF: ADC reads 1.250000 V
The readings are all very repeatable, and stable, which is a good thing. I'm trying to figure out what I have overlooked. Have any ideas? I figure the 2.5V reading is somewhat reasonable with the input grounded, as that is halfway in the voltage range, and the ADC may scale so ground is at Vref/2. What is baffling to me is why a larger voltage reads less?? Does this sound like anything you guys have experienced in the past?

As always, thank you all for your kind assistance!

I'm trying to wrap my head around what I'm seeing. Do you guys have any ideas?

[David Hess]

Below is some readings I have been getting.from the ADC: (Vref = 5.0 V, +In is connected to -In through 10K resistor)
With inputs shorted: ADC reads 2.49997 V
with input connected to VREF: ADC reads 1.250000 V
The readings are all very repeatable, and stable, which is a good thing. I'm trying to figure out what I have overlooked. Have any ideas? I figure the 2.5V reading is somewhat reasonable with the input grounded, as that is halfway in the voltage range, and the ADC may scale so ground is at Vref/2. What is baffling to me is why a larger voltage reads less?? Does this sound like anything you guys have experienced in the past?

I am not completely sure what you are doing but the differential input range is -Vref/2 to +Vref/2.  It does not make any sense to connect the input to Vref as this is outside of the differential input voltage range.  Table 2 in the datasheet says that the output should be +0.000000000 when the differential input is zero.

[Crossphased]

I am not completely sure what you are doing but the differential input range is -Vref/2 to +Vref/2.  It does not make any sense to connect the input to Vref as this is outside of the differential input voltage range.  Table 2 in the datasheet says that the output should be +0.000000000 when the differential input is zero.

I see... perhaps it is due to my own misunderstanding then! Hopefully I didn't damage the chip. Here's the connections I've made:
+Vref = +5 V
-Vref = Ground
+Vin = Vin
-Vin = Ground

I guess I have misunderstood the range of inputs. I was under the impression with a reference of +5V, I could measure inputs between 0-5V. But what I'm understanding from you is that I can measure inputs of-2.5V to +2.5V. Is that the case?