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Transimpedance amplifier voltage and current gain.

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zoulzubazz

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Hi guys,

I am trying to measure a small current (near DC, ~1-10nA) using a 12 bit ADC. The obvious way to do this is using a transimpedance amplifier (TIA). To obtain a mV range output from the TIA, the feedback resistor has to be atleast 1MOhm.
I am worried about the voltage and current noise gain due to the large feedback resistance.
For example, an amplifier with voltage noise as low as 100nV the noise is gonna be 10mV (NoiseGain = 1+1M/Rin, Rin=100 because of timing constrains on the circuit). Is there any way to get around this issue. Is making ratiometric measurement with the ADC a good idea? Thanks very much.
 

The problem description doesn't make much sense yet. You say to measure nA "near DC" current but have 100 ohm Rin "due to timing constraints". These are simply incompatible specifications. It's almost purposeless to drive a TIA with 100 ohm source impedance.

Perhaps you can clarify the problem with a schematic?
 
This article by Bob Pease might help.

**broken link removed**
 
Hi,

if you have a low ohmic source (a current source usually is very high ohmic!) then maybe a voltage amplifier with low voltage noise is the better solution. (LT1028)

If you really want to measure current, then the TIA with a low_current_noise OPAMP might be the better solution. (FET input OPAMP)
Use low noise feedback resistors. (Avoid laser trimmed resistors. Especially I-cut trimmed resistors have increased noise). I tend to micro-melf or mini-melf ones.

But noise always depend on bandwidth. So if you limit upper frequency you will decrease noise. (use a unity gain OPAMP, use a capacitor paralleled to feedback resistor, use an RC lowpass at the output)
"Near DC" means this is a good option.

Look for OPAMPs with low fc in noise figure.
If possible use a single stage solution, multiple OPAMPs will increase overall output noise.
Take care about clean supply voltage and clean bias voltages - if some involved.
Also take care about input capacitance. With large input capacitance sometimes a series resistor in the input of the TIA may improve high frequency noise.


Klaus
 
Hi,

The application is similar to the one mentioned here but with a current much lower than presented there.

https://www.analog.com/en/design-ce...ence-design/circuits-from-the-lab/cn0357.html

I say near DC becuase the sensor response time is typically 30seconds and Rin affects the sensors settling time as mentioned in last paragraph page 2 of the circuit note. An equivalent circuit for the sensor is presented

https://www.alphasense.com/WEB1213/wp-content/uploads/2014/01/AAN_111.pdf

Any tips of keeping noise gain low would be very helpful. Thanks.
 

Hi,

There is no question. So I´m not sure what you need.
All I can think of is answered already.

Please re-read our posts.
Then ask a specific question.

Klaus
 
You should have mentioned EC sensor application before.

There's no problem of OP or resistor current noise due to the low bandwidth (usually < 0.1-1Hz). You'll have a relative large low-pass capacitor parallel to the I/V conversion resistor, as in the linked ADI application.
 
Thanks FvM, This brings two more questions in my mind:

Since this is close to DC wouldnt (DC) offset voltages and current impact the accuracy?

Also since the bandwidth is so low is there any reason a simple resistor in series with the sensor cant be used (get rid of the op amp all together)?

- - - Updated - - -

Thanks FvM, This brings two more questions in my mind:

Since this is close to DC wouldnt (DC) offset voltages and current impact the accuracy?

Also since the bandwidth is so low is there any reason a simple resistor in series with the sensor cant be used (get rid of the op amp all together)?
 

OP offset parameters surely affect accuracy. But it's not very critical for EC sensor amplifiers. FET OPs with moderate offset voltage are usually suited, also bipolar OPs with low bias current.

You don't use a simple resistor as I/V converter because a specific voltage difference between working and reference electrode must be maintained.
 
Right, have done a formal noise analysis on the circuit using TINA and the results seem acceptable.

This raised another question, in any feedback network the noise gain is 1+ Zf/Zin. But As FvM pointed out this effect can be mitigated in this particular (near DC) application with a large cap in the feedback. But what about the DC offset voltage; after all it is still noise at DC. Wouldnt this be multipied by the TIAs gain and appear at the output. This is in clear contradiction with the DC considerations presented in the link (so I know I am wrong in thinking this way) but could someone explain why AC noise gets multiplied by the TIA gain and not the DC noise (offset voltage)? Thanks very much.

ww1.microchip.com/downloads/en/AppNotes/00951a.pdf
 
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Generally, noise gain also applies to the OP offset voltage. But there may be doubts what Zin means. In case of a TIA and an OP with reasonably high input impedance, it's the source DC impedance. It should be relative high for a EC sensor. Or to look on it from a different viewpoint, Vos is added to the sensor reference voltage, causing a slightly different bias point but no additional error current in a first order. The total current measurement error is roughly Vos/Rf.
 
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