Other Parts Discussed in Thread: OPA857
I want to use the OPA857EVM-978 in the photodiode mode with a SM05PD1A Silicon photodiode.
- Evaluate the performance of the OPA857 in the standard photodiode input mode
○ The test mode is disabled: Test_SD = GND, Test_In = +Vs
§ Connected SH-J4 to J4 to set Test_In to +VS
§ Connected SH-J5 to Test_SD (middle pin) and GND
- Measured node voltages
○ Photodiode output voltage: 0.59V
○ Vcc = +3.3V
○ Voltage before C7 (3.9pF): OUT = 3.097V, OUTN = 1.88V, (these voltages are both 1.88V when the photodiode is not connected)
○ Voltage across R8: 2.489V
○ Input voltages to the transformer: V1 = 2.489V, V3 = 2.489V
Transformer output voltages: V6 = V4 = 0V --> VOUTN = VOUTP= 0V
1. Why is the output (OUT) from the OPA857 3.09V? I thought this is optimized to swing below the internal common-mode reference voltage (1.83V).
2. Why is the transformer output 0V?