Reply to thread

TIA is any Op Amp using Vo to Vin- with negative feedback to convert input current to output voltage.  CMOS types tend to be rail to rail in/out so this is your go-to for precision current sensing, while an OTA IC is more about dynamic signal manipulation like measuring phase shift of a frequency from 500 to 5kHz or 5kHz to 50 kHz in a PLL recently asked on this forum.  It could be modified to measure phase of current or voltage and provide a 1 decade sweep or 2 then something else to measure log of amplitude.,

If you could define the range of excitation , V, I , f then I am sure a custom solution can be designed or find something that exists in your low current range.

Low cost meters called RLC meters usually drive constant current at 3 fixed frequencies to measure precise R or L or C with ESR  and R || shunt characteristics.  But for servo circuits people want to see the Bode' amplitude and phase response for voltage or current over several f decades.  But if you want data collection at a fixed frequency RLC meters are the way to go with a PC or uC board kit but you don't get to change it from the fixed options like 120 Hz, 1kHz, 100 kHz or 1MHz

 etc.

There are also simple designs that can generate  constant current (Howland CC cct) or like RLC meters for DC or AC to very low values. then measuring the voltage gives impedance and  mixing the two (V*I) gives phase as a duty cycle or filtered as a DC voltage.

Dana's solution is elegant, powerful  low cost but has a steep learning curve.  You might be able to collaborate with sufficient interest. I don't have his unique expertise or patience.

> 1V 200Hz,  100μA to 100pA, 6 to 20 deg

These two plots are related to linear impedance calculations.

You might get better resolution at 1 kHz with low capacitance readings and high resistance.  If the frequency slope is 1st order or 90 degmax then the ideal measurement is to find the frequency that matches a 45 deg. phase shift and measure the current. This solves both RC immediately and could be done with a  VCO PLL and auto scaled reference capacitors with analog switches.

So if you can make a list of specs like a meter with range specs that may be feasible with ideal parameters of measurement ( impedance ,degrees , conductance ,resistance capacitance half-power point (R=Xc) etc. Series R, Parallel R and error tolerance.

[ATTACH=full]198654[/ATTACH]

[ATTACH=full]198658[/ATTACH]

Then finally if you could buy enough for the whole dept. how many and not to exceed $?  Protoypes can be expensive.