Description
This is a TINA TI simulation of the input stage of the ADC, as described in the Design Manual.
To run it, you need the TINA tool from Texas Instruments, available at: http://www.ti.com/tool/TINA-TI
Notes from Steve Carpenter (designer)
Here is a copy of the performance figures for each of the 4 input attenuators:
Freq response flatness from DC to 200MHz:
- 1M6dB error 0.028dB pk-pk ±0.16%
- 1M26dB error 0.022dB pk-pk ±0.125%
- 50R6dB error 0.022dB pk-pk ±0.125%
- 50R26dB error 0.024dB pk-pk ±0.135%
Freq response at -3dB:
- 1M6dB 1.35GHz
- 1M26dB 1.35GHz
- 50R6dB 1.42GHz
- 50R26dB 1.42GHz
I think the frequency response is possibly too high, and it should be deliberately reduced for a number of reasons. For instance the LMH6518 has a -3dB bandwidth of 900MHz.
The flatness is good, and well within the accepted norm of ±1%. You will notice that I was forced to buffer the outputs of the 50 Ohm attenuators with OPA694 (SOT23 package).
We both need to remember this is a simulation, and the real world version might not be quite as good.