Hi!
We are using six AMC1300B ICs on our board for three pairs of voltage and current measurement channels. Please find the schematics for each of the circuits attached.
We seem to be having two issues:
1) There are three current channels which are fully isolated with respect to each other. The AMC1300B ICs are supplied with +5V at the VDD terminals on the input and output sides with the recommended capacitors. The voltage across the protection diodes show +5V and -5V respectively. The current supply to the shunt resistor is 15A. For no applied input, a common mode of 1.43V is obtained at the output, which is very close to what is desired (1.44V). But when an input AC signal ranging from 10mV(pk-pk) till 250mV(pk-pk) is applied, the AMC1300B provides a differential output signal that is 330mV (pk-pk). The result has been amplified only by a factor of 1.3V/V, which is way lower than 8.2V/V. Other two current channels also show the same behavior, and we are unable to find the root cause for this issue. Is there a fault in our component selection? We have tried replacing the ICs themselves, but it did not solve the issue.
2) The voltage channels are being used to measure the phase voltages of a 3-phase induction machine, and therefore the negative terminal of each channel is connected together to create a neutral point of the machine. At the time of testing, a machine was not connected. The circuit will receive an input of 660VAC (pk-pk), and the resistor values have been chosen according to the component value calculator excel sheet provided for the voltage sensing circuit. On powering up the circuit, measurement shows +5V on the VDD terminals of either side for all three AMC1300B ICs. No signal has been applied at the input. Upon measuring the voltage drop across the protection diodes on the non-inverting input, the upper diode has a voltage drop of 4.88V, while the lower diode has a voltage drop of -5.1V. This phenomenon is not observed for the lower pair of diodes (+5V and -5V respectively). A voltage drop of 113mV is observed across resistor R3, and a voltage drop of 62mV is observed across the input terminals. Upon shorting the input terminals, the observed voltage of 62mV disappears, but the other observations remain consistent. On the positive output terminal, a common mode voltage of 2.0V is observed, while the negative output shows an output voltage of 800mV. Both values show huge deviation from the expected value of 1.44V. This behavior is observed on all three channels. We believe that it maybe be due to bad handling of the input bias current of 30uA, but we are unable to find the root cause and solution for this behavior. Has a similar behavior been experienced before? Is there a mistake that we are making in the selection of components or its supply to them?
Thank you for the help!
HV Side Image:
The excel files for calculating the component values:
Voltage Sensing Circuit Calculator:
/cfs-file/__key/communityserver-discussions-components-files/14/Isolated-Amplifier-Voltage-Sensing-Excel-Calculator-Rev-B.xlsx
Current Sensing Circuit Calculator:
