INA116: INA116 Implementation

Hello Randy,

1 - For our application we are measuring across a high-Z load but 10s of fA input bias up to possibly a 100s of fA or so would be tolerable.  Is there a part anyone would recommend as a replacement?

Actually, the INA116 is our lowest input current instrumentation amplifier. Its low, fA input bias current assures that the bias current won't be an issue in your application. All of our other instrumentation amplifiers have input bias currents int picoamps, or nanoamps, so they would be too high from the start.

2 - If we stick with the INA116 there is of course the guard ring guidance that mist be considered.  We will have the amplifiers close to the point of measurement but reading the section on input connections I was looking at the coaxial configuration and wondered if the shields indicated in the top Fig. 5 (see below) are tied together at the source or if they are floating as seems to be indicated.  The latter seems like it would be susceptible to EMI and such.

It does look like the two input cable guarding circuits shown in Figure 5 are well thought out from a leakage current standpoint. I do believe the cable connections are presented as intended. I suspect that EMI susceptibility may have not been a consideration when the application was being optimized for low leakage currents. It is difficult to know if these cable arrangements would be any more susceptible to radiated EMI than other arrangements. EMI susceptibility mechanisms are not always obvious.

If EMI sensitivity is determined an issue in an application circuit that the best approach is to address the EMI without disturbing the low leakage cable design as much as possible. I do think that adding correctly selected ferrite beads of the proper size to fit over the coaxial/triaxial cables, with an effectively high impedance at the EMI frequency, would be a portion of the solution. Series ferrite chokes at the INA 116 inputs might be an option as well. Adding RF capacitors from the INA116 inputs to ground in conjunction with the ferrite bead inductors would create an RF LC filter. The issue with the capacitors is they would have to have high quality dielectric. There are likely high-quality C0G capacitors with low dielectric leakage. A low-leakage, C0G X2Y capacitor could be added at the INA116 inputs to provide common-mode and differential-mode RF filtering.

Regards, Thomas

Precision Amplifiers Applications Engineering