I'm building a circuit to detect the envelope of a very small signal (some 10s of uV @250kHz, an ultrasound echo. I calculated that I'd need approx 90dB of gain before sending it to a leaky peak detector (and then to ADC: ltc2334-16).
Now I'm having strange or incorrect measurement results:
concerning the picture, apparently scaling did drop of. input signal is scaled at 10mV/div, blue output is single-ended (via RF transformer as in the AD8331 eval board) output, 1V/div.
based on the mechanical return signal I get 20log(4V/20mV) = 46dB
Would you mind sharing the AD8334 portion of your schematic so I can see all of the connections? If possible, can you label the supplies, where you are applying the input, and where you are measuring the output?
If the large mechanical return signal is over-driving the part, calculating the gain based off of that won't be very accurate. Do you have a function generator available to replicate your signal of interest to apply to the inputs instead? If so, the function generator likely won't be able to make a clean 10-100uV signal, so you may need to make a voltage divider to the input of the AD8334 and use a larger signal from the function generator (sine wave should be fine).
I quickly removed the unnecessary schematic parts (not to not show you, yet as otherwise it's much more bloated)https://github.com/devriesewouter89/silense
the pages of main interest are: 1, 3 & 4. I need 4 channels boosted with each channel going through: 1 LNA, 3x VGA.
Therefore on page 1, I first send the first channel to LNA 1, next to VGA 1, next to VGA channel 2 (each time with 100nF caps to connect). The second channel goes to LNA3, then to VGA 3 then to VGA 4. Same goes for channel 3 & 4 (page 3).
In page 4 all amplified channels then enter the last AD8334 each once in a VGA channel.
I'll take a look into the function generator hint!
Thanks a lot
I will take a look at your schematic and see if I can make any other suggestions.
One other thing that came to mind was only connecting one amp at a time and checking to make sure that the output/gain of each is correct to isolate where the issue is. Maybe start with just one VGA, and then add in the others once you have verified that first LNA + VGA stage gain is the correct value.