I have an HCPL-7840 isolation amplifier. It's input side is connected to some high voltage (~300V) power circuitry.
It produces a differential output signal that is nicely isolated from the power side. Each of the two pins that form the signal are between 1.29 V and 3.80 V with respect to the ground of the output side.
I want to take that signal and feed it to the analog components in my MCU, which is an ATXMEGA128A3U. It shares a ground with the output side of the isolation amplifier. And it accepts differential inputs. But it requires that both voltages be between 0 V and 3.3 V, and further that the differential voltage be within +/- 1 V.
How do I glue these things together? I have some understanding of how it is done for single-ended signals, answers to that problem are crowding out the answer to this problem in my search attempts.
I have two uncommitted DACs in the MCU that can output a reference voltage for what I want the new common mode level to be, if it helps.
Best Answer
I think a couple of voltage dividers will fit the bill perfectly. Presuming a 'full-scale' input will produce +1.29 volts on one output and +3.80 volts on the other output (and vise versa for negative full scale), the difference between the two is 3.80 - 1.29 = 2.51 volts. If you scale both output voltages down by a factor of 2.5 to get the differential voltage on the right scale, you get 0.516 and 1.52 volts, both of these within the 0 to 3.3 volt range. So I would suggest simply building two divide-by-2.5 voltage dividers.
Let's assume the output voltages are split into differential and common mode components. In this case, the output voltages Vp and Vn would be Vp = Vc + Vd and Vn = Vc - Vd. If you use decent voltage dividers, the output voltages to the ADC would be Vp,adc = (Vc + Vd) / 2.5 and Vn,adc = (Vc - Vd) / 2.5. Then the converter takes the difference internally, Vadc = Vp,adc - Vn,adc = (Vc + Vd - Vc + Vd) / 2.5 = Vd / 1.25.
Now, if the dividers are not exactly matched, then the result is a little different. Call the divider gains K1 and K2. The ADC output voltage will be Vadc = Vd (K1 + K2) + Vc (K1 - K2). The differential gain is Adm = (K1 + K2) and the common mode to differential gain is Acm,dm = (K1 - K2). In this case, the error caused by the common mode voltage will be proportional to K1 - K2.