SBAA347 June   2022

Design Steps

1. Determine the transfer equation given the input current range and the fixed gain of the isolation amplifier.
2. Determine the maximum shunt resistor value.
3. Determine the minimum shunt resistor power dissipation.
4. To interface with a 3.3V ADC, the AMC3301 and TLV9002 can both operate at 3.3-V supply voltages so a single-supply can be used.
5. Channel 1 of the TLV9002 is used to set the 1.65-V common-mode voltage of the single-ended output of channel 2. With a 3.3-V supply, a simple resistor divider can be used to divide 3.3 V down to 1.65 V. Using 1 kΩ for R2, R1 can be calculated using the following equation.
6. The TLV9002 is a rail to rail operational amplifier. However, the output of the TLV9002 can swing a maximum of 55 mV from its supply rails. To meet this requirement, the single-ended output of the TLV9002 should swing from 55 mV to 3.245 V (3.19 Vpk-pk) .
7. The VOUTP and VOUTN outputs of the AMC3301 are 2.05 Vpk-pk, 180 degrees out of phase, and have a common-mode voltage of 1.44V. Therefore, the differential output is ±2.05 V or 4.1 Vpk-pk.

In order to stay within the output limitations of the TLV9002, the output of the AMC3301 needs to be attenuated by a factor of 3.19/4.1. When R3 = R4 and R5 = R6, the following transfer function for the differential to single-ended stage can be used to calculate R5 and R6.

8. Using our previously calculated output swing of the TLV9002 and choosing R3 and R4 to be 10kΩ, R5 and R6 can be calculated to be 7.78kΩ using the equation below.

Using standard 0.1% resistor values, a 7.77 kΩ can be used. This will provide a maximum output swing within the limitations of the TLV9002.

9. Capacitors C1 and C2 are placed in parallel to resistors R5 and R6 to limit high frequency content. When R5 = R6 and C1 = C2 , the cutoff frequency can be calculated using the following equation.

When the C1 = C2 = 1 nF and R5 = R6 = 7780 Ω, the cutoff frequency can be calculated to be 20.45 kHz.