Measure volatge from a High impedance source

[rakeshm55]

Hi
i am trying to measure voltage from a transducer (rep as JR1). it usually has a high impedance of 20mohms or about. When triggered its impedance will drop to 68k.
I have chosen R1 as 68k so voltage will climb from 0 to 2.5V.(neglecting bias current , offset voltage etc).

Now i want to use another transducer whose value drops from the same (20-30)Mohms to 6Mohms.
So incase i choose R1 as 6.8Mohms will LM358 work or do i need to look for a new opamp. Or
can i measure the value across R1 directly using a microcontroller eliminating the voltage follower ??

Thanks in advance.

[branadic]

First of all, physical units are case sensitive, so you mean 20Mohms

You will need a buffer with high input impedance, because the input impedance is in parallel to your transducer. Using a buffer with 100x high input impedance (20Gohms) compared to your transducer will result in 1% error.
So you better chose a buffer with FET input stage and input impedance in the range of >100Gohms. Texas instruments is your friend.

[alm]

As branidac mentions, 20 mohms is only 0.02 Ohm, that's not very high impedance. You can just use a resistive divider and directly connect it to the ADC.

If you actually meant 20 MOhm, then input bias currents of bipolar opamps will swamp the signal from the ADC. Remember they told you that no current flows into the input of an opamp? That's a lie, especially for opamps with bipolar inputs. I would also suggest a FET buffer.

The Keithley low level measurements handbook is a valuable resource for these kinds of problems, and will mention some of the issues you're likely to encounter at the ridiculously low currents that you're likely to be working at.

[rakeshm55]

Hi
Though the transducer impedance may be 20Mega Ohms. I have  resistor R1 connected to ground.which is 6Mega ohms
ie
5V-----(20Mega ohms) -------(R1 to ground)
I am measuring voltage across R1 , so the o/p impedance of the combination be R1||20Mohms right??
So in case when R1 changes to 6Mohms the impedance would be 4.6Mega Ohms.

Now I have a (bias current x 4.6Mohms) voltage the o/p of opamp. Is this correct??
How to get the i/p impedance of opamp

[Mechatrommer]

FET opamp is your friend. TL071 series opamp has 1Tohm input impedance, more than adequate for a few cents.

[Smokey]

Pick a buffer opamp with really low input offset and really low drift if you are going to be measuring really small signals.  Especially if this is a one-off board and not something for volume production.  Spend the money up front for a really really nice opamp instead of using whatever is in your junk drawer.

[jerry507]

Unit names are case sensitive too. Ohms are named after a person and are also capitalized

[Phaedrus]

You can skip the Ohms part though, and just use M. If it's clear you're talking about impedance then no one will mind, but if you're mixing units you need to make sure to spell them out correctly and capitalize.

Love,
Herr Grammar (SS)

[ejeffrey]

Unit names are case sensitive too. Ohms are named after a person and are also capitalized

Not correct.  SI units named after people should be written as lower case (at least in English).  However, their abbreviations are capitalized.  1 tesla = 1 T, 1 ampere = 1 A, 1 ohm = 1 <uppercase omega>.  Units that are not named after people are lower case in both the word and symbol form: 1 gram = 1 g, 1 meter = 1 m.

[AlfBaz]

... and when dealing with spice m, capital or other wise is milli, you have to type meg