• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 49 subscribers
  • Views 401 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

OPA365: ADC buffer for tuner IF differential to single end, with automatic gain control,

BIDAULT Hoel
Intellectual 320 points
Part Number: OPA365
Other Parts Discussed in Thread: VCA810, VCA820, VCA2612

Hello,

As a follow up to previous quoestion, i am using an OPA365 to drive the ADC of an STM32F411CEU6 (12bit, single end)from the output of a tuner providing an IF which is between ±640KHz and 800KHz.

Currently there is a 4:1 balanced-unbalanced impedance transformer at the tuner output (which is differential, with a max amplitude of 2.0V PP).

The OPA365 is used to offset by 1.65V and provide the gain to reach (close to) full scale of the ADC. This setup is working but there is a number of problems and it is probably suboptimal, first of course the output is not exactly rail to rail, which mean some codes are lost at the bottom of the ADC range, however it is not the most major issue, a more conerning problem is that the OPA is prone to saturation and as soon as the tuner VGA gain is too high it will saturate by "folding" on top and clipping at bottom. Screenshot attached.

The major problem i have is that the digital processing chain behind the ADC can only compensate for a loss of 3bits in resolution, so as soon as the absolute value out of the adc is less then ±512 it will not demodulate correctly, i already set the tuner LNA and VGA gain to highest settings, and the tuner AGC is also enabled.

This lead me to the question : 

-Is there a better way to do what is currently done by the combo impedance transformer + OPA365?

-Is there a way to either add a control loop to the OPA365 or add another OPA after it to act as an AGC? i would like to keep feeding the ADC and the processing chain within it ideal range (that is over 9bit resolution), as long as possible( until the SNR is too low anyway).

Thanks.

  • 0
    Guru 47485 points

    That is truly a homely waveform, you might try reducing your R values around the op amp by 10X

  • 0 in reply to Michael Steffes
    Intellectual 320 points

    thank you for the return,

    Yes the values are too high indeed, i'll reduce to 8K and 12K, however it is a minor problem since this only occurs if the VGA gain is too high, which is rather the contrary to what occurs in the field (sadly). 
    What i am really looking after is the automatic gain control, is there a way to make a control loop with this OPA directly, or with a second OPA before or after this one to make a gain control loop?

    I have seen the AD603, which can offer up to 51dB and can be used as AGC, but it is rather bulky (SOP8) and space it at premium on the PCB, i dont know if there is something similar in the TI portfolio.

  • 0 in reply to BIDAULT Hoel
    Guru 47485 points

    Well just responding tothe AGC part, 

    the dB/V gain control versions are what you want if you are building your own AGC - the VCA820 describes this but is likely too fast for what you need. You might also look at the VCA810 for a slower design point 

    There is also that range of ultrasound VCA's in the VCA2612 type parts - those are normally multi-channel. Those are not so much servo loop but gain ramp