Picoammeter Design

[3roomlab]

i have a noob question, after seeing this thread and this http://www.vk2zay.net/article/251
yates' version drives a LCD readout, i was wondering if this a good way to buffer the LMC662 to drive something else like a cheap LED readout sprawled all over ebay?

[dom0]

If you want to filter incoming noise from the meter inputs I'd say a T-filter network is a better choice. Assuming, of course, there is such noise, and that it affects the amplifier.

[3roomlab]

im not sure about noise, i just happen to have those standalone 5 digit LED volt display. so i am thinking of the unit as stand alone w/o requiring a DMM to read it. the input impedance is few xxxk so i dont think i can connect direct to the LMC

[dreaquil]

You should be able to hook it up directly. The LMC662 is specified for 2k and 600ohm loads. It also has a minimum output current of 14mA for the AI version so it should be just fine.

[Gyro]

The advantage of using a DMM is that you get range switching - you can sense to just over +/-4nA (+/-4V out) using the 1G feedback resistor and with a fresh 9V battery. With a 5 digit display however that isn't as much of an issue.

The LM662 should drive that sort of load without any issues. Remember though that the output is inverting (-ve for positive input) and it is biased mid way between the battery supply rails (mid rail bias is provided by the other half of the 662) so you need to make sure that the DVM module can handle that if you're planning to use the same supply rails as the 662.

[Tartan5]

Hi everyone,

I am looking to make a board that includes multiple (50 or more) TIA amplifiers.  I'd like to be able to resolve 0.1 nA DC.  How well would the LMC662 work from two linear regulators (noise wise) vs a battery?

Thanks!

[Gyro]

Hi Tartan5,

It should work quite well. I found that with battery operation the input offset voltage does shift over the battery voltage range (mind you I'm talking a 2:1 range there), which regulated supplies would prevent.

Yes, you will need to be careful of noise, but the supply current will be low so you can easily apply good filtering. Also be careful to avoid ground loops.

[bdivi]

Gyro,

What was the reason to use split supply instead of single ?

LMC662 datasheet specifies that the input should work to ground and the output is rail to rail.

Thanks,

[Gyro]

Hi,
The split rail is so that I don't have to worry about input polarity. It will do +/- 4.5nA full scale on a fresh battery (+/- 2.5nA by change time). On a picoammeter with BNC input connector it's not a simple as swapping the input leads, the outer is effectively ground.

[bdivi]

I understand - thank you.

Regarding the split supply in your schematics. Is it the virtual ground buffer with resistor in the feedback and capacitor to ground ? The way it is now it is some sort of an integrator and does not make sense to me.

[Gyro]

They are there (together with the resistor on the output) for stability, the rail splitter half of the LMC662 has a tendency to oscillate without them, it is basically a voltage follower at DC though.

[zlymex]

I made two similar ones before, battery powered, split supply and air wiring at input pin of the opamp. I use 100G and 1T as feedback resistors to achieve lower current noise. Also, I use a piece of twisted PTFE insulated wire pair as the feedback capacitor.
I tried different opamps and different resistors, they are not all good.

[mmagin]

zlymex: What is this cute MX-6.5 you have there?  Homebrew or commercial?

[zlymex]

zlymex: What is this cute MX-6.5 you have there?  Homebrew or commercial?

Homebrewed hand held 6 and half digits.
http://bbs.38hot.net/thread-4594-1-1.html

[dannyf]

I have done quite a few experiments with a Pico ammeter built around cmos opamps, in topology almost identical what what's shown earlier. If you do a search you will see those posts. Mine was inspired by a keithley meter.

The biggest lessons I learned is that the use of a t-network, often seen in a mature and academic papers, is a crime in building those meters.

I ended up constructing a Na meter around a tl0x4 chip, as an adapter to a digital multimeter with relative measurement capabilities.

I think I tried quite a few of lose cmos opamps mentioned here with fa-level bias current.

[dannyf]

"The LMC662 in dip pkg looks a very good bet for avoiding a PCB and associated leakage,"

Unless you have access to super duper PCB with looooow leakage, point to point wiring is the only way to go. I used some aerospace grade wires in my built.

The most critical component in this type of build is that resistor. Very difficult to get a good resistor into the T or Gohm territory.

[mmagin]

zlymex: What is this cute MX-6.5 you have there?  Homebrew or commercial?

Homebrewed hand held 6 and half digits.
http://bbs.38hot.net/thread-4594-1-1.html

Cool.
May I ask what voltage reference and ADC you used?

[zlymex]

Cool.
May I ask what voltage reference and ADC you used?

Vref: ADR441
ADC: LTC2440

[bdivi]

I built the Gyro picometer myself and I am very impressed with the performance.

Once nulled and quiet it stays close to 0.000 mV and only moves +/- 3 uV which is 3 fA 

100mV with 100MOhm resistor measures 1000.2 pA - again pretty nice for 1% 1GOhm feedback resistor.

Very happy with this project

[David Hess]

Back when I was doing this, we graded LMC6081s for low input current and I do no remember finding any which were above 1 picoamp.  Using the DIP package, we bent the inverting input straight out and air wired it.

There was a Burr-Brown application note about a picoamp current inverter which could have been used as the basis of a picoamp input transimpedance amplifier; notably it used low leakage diodes in place of the high value feedback resistors.  In his book, Bob Pease described a similar circuit for making picoamp measurements; in his case, the logarithmic output was a feature.

Good 10 megohm input digital voltmeters can achieve 10 picoamp resolution or better.

[Tartan5]

I made two similar ones before, battery powered, split supply and air wiring at input pin of the opamp. I use 100G and 1T as feedback resistors to achieve lower current noise. Also, I use a piece of twisted PTFE insulated wire pair as the feedback capacitor.
I tried different opamps and different resistors, they are not all good.

Hi Zlymex,

Could you list what feedback resistors (mfg pn) you were testing?  I am trying to find a good source of low noise high resistance resistors to use for the feedback resistor.  I've heard that the thick film resistors tend to have quite a bit of inherent noise...

All:  Overall, what style of resistor seems to perform best for this application?

Thanks!
Tartan5

[zlymex]

Hi Zlymex,

Could you list what feedback resistors (mfg pn) you were testing?  I am trying to find a good source of low noise high resistance resistors to use for the feedback resistor.  I've heard that the thick film resistors tend to have quite a bit of inherent noise...

All:  Overall, what style of resistor seems to perform best for this application?

Thanks!
Tartan5

Top two in the first photo are glass-vacuum type, the one I tested are average. The quality may depend on manufactures a lot.
Next two(red) are film resistors made in China, good quality, I bought twice, they are my main resistors for week current measurement
The dark brown one is made in Japan, perform good in high voltage but very poor at low voltage as exhibit very large DA(Dielectric Absorption) effect
Bottom two are very bad, too much voltage co-efficient

I also tested some Ohmite resistors such as mini-mox(mox1125-23) and slim-mox(102E and 204E), good for this purpose.

Photo only show my 100G resistors, I got other decade values(100M to 1T) but looks similar in appearance.

Edit: Even the performance from the same batch is different, this especially true for high resistors such as 100G and 1T. For value 10G or below, it is not critical.

[bdivi]

The Ohmite 104E I used is also very good - 1GOhm, 1% tollerance (happened to be better than 0.2%), TCR 25ppm/C, 0.25% voltage coefficient to full voltage rating and 0.25% humidity variation.

http://www.newark.com/ohmite/sm104031007fe/thick-film-resistor-1gohm-1-5w/dp/66K6874?ost=SM104031007FE&selectedCategoryId=&categoryNameResp=All%2BCategories&iscrfnonsku=false

[Tartan5]

Thanks Guys.  I'll get a few different ones on order.  I plan on laying out a few different op amp / resistor configurations on a test PCB.  I am going to try some surface mount feedback resistors with guarding as well.....

Thanks!
Tartan5

[Tartan5]

Quick question..... wouldn't the thermal noise of the 1G or 10G resistor swamp out the current reading?  I know I can LPF the output, but what if I wanted to sample at 10 hz or so?

Thanks!