1
\$\begingroup\$

I'm wanting a to increase the input impedance to a differential ADC. The ADC spec states that it can't handle a negative voltage lower than -100mV the ADC's supply can handle ~5V (this is a ATtiny1617)

I assume this means that the differential input works as follows into the IN+ and IN- pins of the ADC? IN+ = 0v & IN- = 0-5v, this would be the negative range of the ADC IN+ = 0-5v & IN- = 0v, this would be the positive range of the ADC

Assuming the above is correct. I need to buffer the input to increase the input impedance.

I've simulated the following circuit and it works fine, however I don't want R2 & R3 on the input as this reduced the input impedance, but if I remove these the simulation doesn't work.

The simulation also shows that X1-out & X2-inn swing negative. How should this be resolved?

enter image description here

enter image description here

Below with R2 & R3 removed

enter image description here

\$\endgroup\$
5
  • \$\begingroup\$ Can you explain why is 10Mohm not good enough for you? Also, if you're biasing your input signals at 0V, why don't you AC couple your inputs? \$\endgroup\$
    – Designalog
    Feb 26 at 22:11
  • \$\begingroup\$ First things first- it is necessary to ensure that both inputs remain within the common-mode input range of the op-amp (in the case of the MCP6001, approximately within the supply voltage rails). How best to do that depends on where the voltage is coming from. For example, you could ground one side of the source and get GΩ input Z, but then you wouldn't need a differential amplifier. \$\endgroup\$
    – Spehro Pefhany
    Feb 26 at 22:54
  • \$\begingroup\$ @ErnestoG the sensor this will interface is very high impedance. Its a PH sensor. \$\endgroup\$
    – John
    Feb 27 at 1:35
  • \$\begingroup\$ @John you should add that info in your question. Source impedance is a very important piece of information in any amplifier design. \$\endgroup\$
    – Designalog
    Feb 27 at 9:35
  • \$\begingroup\$ related: Differential ADC input buffer \$\endgroup\$
    – davidcary
    Aug 3 at 16:36

1 Answer 1

Reset to default
1
\$\begingroup\$

You can use an INA or INstrument Amp IC to create the Gohm differential interface with a common external pair of gain resistors that are well matched and use that common mode signal inverted to feed back to the solution like “Right Leg Drive” RLD does for ECG/EKG or EEG circuits use to suppress interference then drive the ADC with a differential amp well-designed or use STP cable single-ended. All must be somehow DC biased to the middle of the CM range. I suggest you consider those solutions but bypass the C coupling.

Proximity to sensor is important as well as choice of “ground” = 0V for DC high impedance sensor and EMI for AC reduction just like in EEG microvolt 1 Hz signals. This requires a precise level shift from 0V = null on input to 2.5V = null on ADC. which is added to the INA as a Vref or the next stage.

The EEG prefilters are also useful for you to suppress line-induced Efield voltages and AM radio RF.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.