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Design >> Mixed-Signal Design >> Using an attenuator after the BPF attached to the sinewave gen. in ADC Testing
https://designers-guide.org/forum/YaBB.pl?num=1437076327

Message started by DesertFox on Jul 16^th, 2015, 12:52pm

Title: Using an attenuator after the BPF attached to the sinewave gen. in ADC Testing
Post by DesertFox on Jul 16^th, 2015, 12:52pm

Hi,

For generating low distortion single tone for testing ADCs, the "Data Conversion Handbook" suggests using a 6dB attenuator after the BPF/LPF connected to the sinewave generator, as it isolates the ADC from the output of the filter. Can anybody tell me why I would need this, as long as I use a termination resistor of 50ohm at the input of the ADC? I am attaching the figure.

Thanks

Title: Re: Using an attenuator after the BPF attached to the sinewave gen. in ADC Testing
Post by loose-electron on Jul 19^th, 2015, 1:55pm

No good reason seen - they just seem to be accounting for the termination impedance and its presence as a voltage divider perhaps? (That's speculation)

Title: Re: Using an attenuator after the BPF attached to the sinewave gen. in ADC Testing
Post by DesertFox on Jul 19^th, 2015, 2:43pm

Rt is working as the termination resistor. All the book says is the 6dB attenuator isolates the ADC from the output of the filter. I am not sure what is the reason for isolating, or why they are isolating.

Title: Re: Using an attenuator after the BPF attached to the sinewave gen. in ADC Testing
Post by loose-electron on Jul 20^th, 2015, 10:54pm

books have mistakes and say incorrect things. I do not see a need for it either.

Title: Re: Using an attenuator after the BPF attached to the sinewave gen. in ADC Testing
Post by nrk1 on Jul 21^st, 2015, 7:11pm

If the total impedance presented to the filter is exactly 50 ohm, the attenuator does nothing. But, with errors in termination, say due to input cap of dut, attenuator reduces mismatch for the filter. This maybe a concern for the response of the passive filter's response. Also, it reduces the signals (say kickback) from the adc going back into the filter/signal generator.

DesertFox wrote on Jul 16^th, 2015, 12:52pm:

Title: Re: Using an attenuator after the BPF attached to the sinewave gen. in ADC Testing
Post by rfmagic on Aug 3^rd, 2015, 1:19pm

I agree with nrk1. The sole purpose of the attenuator is to account for any mismatches due to not perfect 50 Ohm termination.
In most cases PCB designers add these attenuators automatically without really paying attention if they are required or not.

Title: Re: Using an attenuator after the BPF attached to the sinewave gen. in ADC Testing
Post by aaron_do on Aug 4^th, 2015, 7:04am

Hi all,

as I recall, we did this all the time when measuring switch non-linearity. Because the linearity requirements were so tight, it was important that the generator not be affected by its load. So I would add to what others have said that it makes sure the load presented to the generator is linear. The other point by nrk1 is also important, that it isolates the generator from any kind of kick-back signals. Remember that the generator itself is some kind of active circuit with its own IIP3 etc...

regards,
Aaron