Instrumentation Amplifier recommendation for use with Photo diode low light reading

Paul Evans

Other Parts Discussed in Thread: ADS1231, INA317, INA111, INA333, OPA392, TLC2652

I am designing a piece of medical test equipment to detect a low light source using a photo diode and am looking for a recommended instrumentation amplifier and ADC pairing for that application.

The result of the opamp will go to an ADC to allow a microprocessor to analyse the light data.

Ideally single voltage source to keep overall design simpler but not essential.

ADC would be 24bit ideally, 16bit minimum.

Ideally differential interface between INA and ADC to minimise noise.

Readings will be taken max 10Hz so very low bandwidth design, just needs to read light from a small fluorescent sample so needs to be sensitive to small changes in the photo diode.

Ideally if there is an eval board for at least the INA side that would allow early prototyping to check suitability.

There are so many different types of INA I would appreciate any recommendations to help target a few types for trials.

Thanks in advance

10 months ago

0 Alexander Smith 10 months ago

TI__Mastermind 48610 points

Hi Paul,

I've modified the selection table to show a few options that may work:

24 bit, less than 10ksps, and one input channel. ADS1231 may fit your needs.

https://www.ti.com/data-converters/adc-circuit/precision-adcs/products.html#84=24%3B24&1028=1%3B1&157max=0.015%3B10&sort=1130;asc&

I'll move this thread to the sensing team for assistance with selecting an INA.

0 Art Kay 10 months ago

TI__Mastermind 18037 points

Paul,

One option is the INA EVM Landing page . The users guide is fairly detailed to explain how this can be used: INA EVM users guide

Another possibility for an evaluating any device is to use dip adaptor cards and hand wired discrete components: https://www.ti.com/tool/DIP-ADAPTER-EVM

I hope this helps.

Art

0 Paul Evans 10 months ago in reply to Art Kay

Prodigy 20 points

Thank you both for your prompt answers. The INA EVM should be useful. Do you have any recommendations on the most suitable parts to start with using this board though?

0 Clemens Ladisch 10 months ago in reply to Paul Evans

Guru 257270 points

You want low input offset voltage and low input bias current. The INA333 has 25 µV/200 pA, the INA317, 75 µV/200 pA, the INA111, 500 µV/20 pA. (The INA111 does not fit on the EVM.)

What transimpedance do you need? (INA333/317: 2 × 50 kΩ; INA111: 2 × 25 kΩ)

0 Paul Evans 10 months ago in reply to Clemens Ladisch

Prodigy 20 points

Not sure on the transimpedance yet but I will look at both the INA333/317 and INA111 now. Thank you all for the suggestions. Very much appreciated and gives me a good starting point. I will get some EVM boards and parts ordered and start prototyping ready for the full design.

Thanks again and have a good weekend.

0 Clemens Ladisch 10 months ago in reply to Paul Evans

Guru 257270 points

When using an INA as a transimpedance amplifier (pins 2+3 shorted and connected to some bias voltage, photodiode in place of R_G), the photocurrent flows through both input feedback resistors, so the output voltage is REF + I_photo × 2 × 50 kΩ. If you wanted a different transimpedance, you would need to choose a different INA, but this is not a parameter that can be easily searched for.

0 Paul Evans 10 months ago in reply to Clemens Ladisch

Prodigy 20 points

Thanks for that advice. Looking at the photo diode I am currently planning to use, dark current is max 30pA and full light current is typically 4.5uA. I believe I will need a higher transimpedance that this then since I am going to be looking at quite low light sources.

0 Clemens Ladisch 10 months ago in reply to Paul Evans

Guru 257270 points

The INA333 would map 5 µA to 0.5 V. If you wanted 5 V, you would need a transimpedance in the order of 1 MΩ, but I doubt that there is any such INA.

But why do you want to use an INA? You could build a plain transimpedance amplifier with an opamp like the OPA392 (low input bias current) or TLC2652 (low offset voltage).

0 Art Kay 10 months ago in reply to Paul Evans

TI__Mastermind 18037 points

Paul,

You probably want to build the input stage discretely if you want higher transimpedance gain. Instrumentation amplifiers normally have relatively low feedback resistors to minimize thermal noise for the typical voltage amplifier use case. You could build the input discretely with two op amps and two resistors then follow the input stage with a differential amplifier or instrumentation amplifier. Also, if the ADC has a differential input you can simply connect the output of the two op amps directly into the ADC.

There are many good choices for low bias current op amps. One possibility is https://www.ti.com/product/OPA2391 . This is a dual op am with very low bias current. It is really intended for this kind of application.

Best regards,

Art

0 Paul Evans 10 months ago in reply to Art Kay

Prodigy 20 points

I had originally thought INA for the accuracy/low noise side but if they all have the lower gain then I will follow your suggestion . Thank you very much for all your support. My expertise is in digital and high speed and analog is my weak area so very much appreciated.

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Instrumentation Amplifier recommendation for use with Photo diode low light reading