Other Parts Discussed in Thread: THS4531A, THS4561, OPA2365
Hello
I have an RF tuner providing differential 2.0V PP output, which i need to sample with the ADC of an STM32L4, this ADC can only accept positive voltage and the package i used (QFN48) does not give access to VREF+ and VREF-, they are internally tied to VCCA and AGND, and AVCC has to be 3.3V. Thus the ADC REF can only be 3.3V. The ADC can be used as single-end or differential input, depending on the configuration, but in any case the input cannot be negative with respect to STM32L4 GND. There is 3 problems here, balanced to unbalanced conversion, amplitude scaling and DC biasing.
The first issue is sorted by using an impedance transfomer 4:1 which has balanced input and unbalanced output.
The second issue could be sorted with the OPA365 which leads to the third issue : DC bias.
Of course the signal coming out of the transformer is swinging over and below 0V, 2.0V PP max. If i directly connect one transformer output to GND and the other to OPA365 i end up clipping the whole bottom half of the wave, which is a problem since the system does demodulation of an RF signal. So the only option here seems to be : DC biasing the OPA365 at 1.65V so that it provide the full sine wave swinging between 0V and 3.3V (ideally), however i fail to even simulate that in TINA. Is there some advice on how i should proceed with this? Of course if another approach is better i am open to suggestion, especially if i can get rid of the transformer, keeping in mind that the design has some contraints, mainly : available space and BOM cost. Basically the less parts we have between the tuner and the ADC the better.
Kepping the signal differential all along including to ADC would be ideal, but i can get to that in a second time, for now i am mainly trying to achieve the DC biasing with single end on the current test board, which only has one OPA365.
Thanks.