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

[foxOnTheRun]

I'm puzzled by this application:


**broken link removed**



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.

 

[chuckey]

Lets take a worst case, motor voltage drop = 500V. Then if you use a conventional voltage attenuator between the op amp inputs, say 10 M in series and 200K between the two inputs (100 :1 attenuation), then the common mode voltage is 250 V, quite a problem! You can get instrumentation amplifiers that can handle this voltage. If you put in two attenuators each returned to earth, 10M in series and 100K from each input to earth then the common mode voltage is the attenuated voltage drop across the shunt, which I hope is extremely low. Have I missed something?
Frank

 

[foxOnTheRun]

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


**broken link removed**

My doubt was where I had to connect the input divider network and the opamp pins without frying it; with this configuration I can read whatever voltage inside the circuit (inside the common mode range of the opamp) just like with a multimeter without isolation issues.

I know, the maximum source voltage is under 42V, but I liked to put inside some extra design margin for the future(of course I'm going to rescale the divider ratio); Looks good to you too?