High voltages reading, differential amplifier and Vcm (AD629, INA148)

[foxOnTheRun]

I'm puzzled by this application


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I would like to be able to measure voltages across sections of this circuit, it has a separate common, and my probe points will sense voltage drop across various segments of it (just like I would use a multimeter).

I simplyfied it by putting 3 resistors to simulate voltage drops (R1 might be mosfets drop, R2 motor drop, R3 a shunt resistor) and a medium 200V Dc voltage source (far high from opam supply). I think I need a diff. opamp with a high Common Mode Voltage but:

_ I think I'm pretty ok if I should measure voltage drop across a shunt resistor (R3), I can connect +V and -V of the opamp across the shunt resistor and I get a Vout = (+V) - (-V) referred to the uP board ground (then I can amplify it and so on).

_ what I miss (somewhat) is how can I read the drop across R2? its voltage range span well outside the opamp power supply rails (this problem is no more a common voltage problem), can I use a normal resistive partitioner to reduce the amplitude of the signal, how would I build the input network (or not)?

Can you point me application notes? until now I can only find examples of reading a shunt resistor, or (ina148 datasheet) they cheat, and use a separate power supply rails to supply the input shaping opamp (but again, they read a simple shunt).

Thank you all,
James.

 

[ronsimpson]

You will need a gain of 1/40 to get 200V=5V.
The out put will be referenced to gnd.

 

[foxOnTheRun]

I built this circuit and ran the simulation: it seems it works fine


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My doubt was where I would connect the input network and the opamp pins without frying it; with this configuration I can read whatever voltage inside the circuit just like with a multimeter without isolation issues.