| Electronics > Beginners |
| High voltage measurement of quasi isolated power supply |
| (1/1) |
| Momchilo:
Hi, I have to measure voltages of an galvanic isolated system with ADCs. The V_IN voltage is around 70V DC or 50V AC. The idea is a difference measurement. It's not galvanic isolated anymore, but isolated through the high input impedance. So it's quasi isolated like differential probes for scopes. In Spice simulations it works very well. Do you think this will work with an isolated V_IN without destroying anything even at higher voltages (<200V) or make it dangerous for a person? Because the other grounds in the schematic are the analog grounds for the amplifiers and the ADCs. It's connected to the digital ground in one point. If you think, this will work, I'm unsure where to connect pin 1 of the AD629. Should it be connected to the analog ground or the isolated ground? I guess to the isolated one. Thanks for your advice. Best regards Momchilo |
| David Hess:
Yes, it should work but where did you find that circuit? The OP177 integrator is there to remove the DC component from the AD629's output without using input AC coupling capacitors which are problematic. The result is that the AD629's output has a high pass characteristic and the OP177's output has a low pass characteristic. Is that what you wanted? In normal applications, both pin 1 and pin 5 of the AD629 goes to the same output ground. In this case, I think pin 1 should go the same ground as pin 3 of the OP177. There is no direction connection to the isolated ground except through the device. |
| Momchilo:
Thanks for the answer, you helped me a lot :) I found this in "A Designer's Guide to Instrumentation Amplifiers" released by Analog Devices. You can download the whole PDF here. The used circuit with a short description is on page 92. How would you change the circuit? At least in the PSpice simulation it works really well up to 100kHz. There is only 1dB attenuation in the output signal at 100kHz. That is more than enough for the measurements. Then I will connect all grounds (except the isolated one) to analog ground. |
| David Hess:
--- Quote from: Momchilo on July 26, 2019, 04:58:28 am ---I found this in "A Designer's Guide to Instrumentation Amplifiers" released by Analog Devices. You can download the whole PDF here. The used circuit with a short description is on page 92. --- End quote --- Ah, I think I remember an earlier edition of that book. Analog Devices has released some of the best documentation over the years. --- Quote ---How would you change the circuit? --- End quote --- I might not change it at all. it is one of the better options for your requirements. --- Quote ---At least in the PSpice simulation it works really well up to 100kHz. There is only 1dB attenuation in the output signal at 100kHz. That is more than enough for the measurements. Then I will connect all grounds (except the isolated one) to analog ground. --- End quote --- The value of C1 can be increased to lower the bandwidth which may be important to control noise. Attenuating input instrumentation amplifiers are pretty noisy. |
| Navigation |
| Message Index |