I designed the circuit below after a suggestion that I use two gain stages rather than one large stage with an expensive opamp. This help keep the cost of my circuit down (the same application has been discussed in my previous questions).
My schematic is:
My thinking when designing the circuit was:
- U1 buffers the Vcc/2 virtual ground (perhaps unnecessary to buffer it, but not harmful)
- U3 should match the input impedance of 52 ohms and amplify x10 (OPA355 datasheet)
- U4 should amplify a further x10
The input signal is 5Mhz and the circuit is on a PCB, seen below.
So for a small input of 10mVpp I expect approximately 1Vpp out. I'll probably reduce the overall gain once I've done some testing but x100 is a good place to start testing.
With a 10mVpp input from a signal generator, checking TP2 with a scope shows virtually the same signal as the input. Testing at TP3 shows a signal x10 larger than the input.
I assume I must have made a very silly basic error. Therefore my question is: can somebody point out what is wrong with the circuit which results in only x10 overall gain.
Note I am not currently testing the comparator (U2), as I haven't yet integrated the data slicer discussed in my previous question. Also I am using the same scope probe for all measurements, so I'm not mistaking x1/x10 settings by accident.