instrumentation amplifier with [2 opamp + ADC] vs [3 opamp + ADC]

I have an 16bit ADC able to do differential measurement with range -5V to +5V (ADS1115), do you think both circuits would work?

It's required that the adc input is within -5V and +5V range, else it would fry.

I think the 3-OPAMP circuit has the advantage that output would be always -5..+5, so no risk for the ADC. I would use a basic OP-07C opamp for this.

Note: I didn't draw the RC circuits.

• Why build a discrete instrumentation amplifier out of op amps when you can buy an integrated instrumentation amplifier?
– Null
Aug 5 '19 at 14:28
• If your opamps have +/-5 V rails, then the output won't be above 5 V. The datasheet for the OP07C shows the peak output voltage with 15 V rails as 13 V, which suggests you'd get +/- ~3 V with 5 V rails. Aug 5 '19 at 14:30
• @Null you are right, but where I live there are no in-amp with low bias current (I need few fA), but I have opamp with low bias current. Aug 5 '19 at 14:35
• @Colin good point, I need higher voltage for +Vs and -Vs at the OP07, I would use +7.5V and -7.5V rails, so output would fine in range. Aug 5 '19 at 14:37

It's required that the adc input is within -5V and +5V range, else it would fry.

That's not quite true.

You should be able to capitalize on the following part of the data sheet specification: -

Input current, continuous (any pin except power supply pins) –10 to +10 mA


Given also that the input range is: -

Analog input voltage  GND – 0.3 V to VDD + 0.3 V


You have a good scope to implement a current limit (and therefore voltage limit) by using a series resistor on your input pins. For instance, if the peak voltage at your input could be as high as (say) 8 volts, a current limit resistor of 1 kohm would produce a current into the ADC input of (8-5.3) volts / 1000 ohms = 2.7 mA i.e. significantly below the 10 mA limit.

I'm not saying you do not need an InAmp to get good circuit performance, but you don't necessarily need one for protecting the ADC.

• Dear Andy, I do have a 10K resistor (and 68uF as RC filter) at the input of the ADC - not represented. So input current cannot exceed 0.5mA, far below the limit. You suggest it could work ok only with 2 opamp+ADC? As written in the comment, I can't find inamp with only few fA of input bias current but opamp are easy to find. Aug 5 '19 at 15:17
• I'm saying that you don't need an InAmp to get protection you think you might need. Aug 5 '19 at 15:39
• ok, as long there is the 10K in serie with the input of the ADC, it's protected. So both schematics are equal, better for me to not bother with the additional OP07. Aug 5 '19 at 15:42

You add current limiting resistor on opamp output and diode clamps.

simulate this circuit – Schematic created using CircuitLab

• Last time I used clamping diode (1N4148), I noticed the output is modified once Opamp output is near but below clamping voltage. I suppose there is a safe range to use where output isn't affected by the clamping diodes. If clamping is -5/+5V, what is working range where output isn't affected? (-4 to +4?) Aug 5 '19 at 17:42
• @Vinlar You clamp the voltage to the ADC input range, the supply of opamp is higher you said +/-7.5V Aug 6 '19 at 8:43