ADC08D1520: ADC08D1520, Q+ Q- input connections

Art Del Castillo

Part Number: ADC08D1520

To Whom it may concern:

We are currently using ADC08D1520 and have configured it as follows:

1). I+/- inputs are ac coupled
2). single ended.

What is the proper way to connect the Q+/- inputs with respect to Vcmo if they are not being used? Please provide a diagram.

Thanks

Art

over 8 years ago

0 Jim Brinkhurst1 over 8 years ago

TI__Mastermind 44680 points

Hi Art

I would like to get a little more information about your device configuration before answering the question regarding the Q+/- input connections.

1) Can you explain what you mean by "single ended". The analog inputs of this device should be driven with balanced differential signal. Performance can be significantly degraded if only one of the differential inputs is driven with a signal.

2) If you are AC-coupling the I+/- inputs I assume you are connecting the Vcmo pin to ground through a maximum of 100 ohm resistance. Is that correct?

3) Since you are only using the I inputs, I assume you are operating the device in DES (Dual Edge Sampling) mode. Is that correct?

Best regards,

Jim B

0 Art Del Castillo over 8 years ago in reply to Jim Brinkhurst1

Intellectual 350 points

Hi Jim!

I believe all your questions are answered in the enclosed picture. So, is this the correct way to connect the unused Q+, Q- inputs with respect to Vcmo?

0 Jim Brinkhurst1 over 8 years ago in reply to Art Del Castillo

TI__Mastermind 44680 points

Hi Art

There is a description of the proper terminations for unused inputs when using ac-coupled mode near the bottom of page 44 in the ADC08D1520 datasheet.

Instead of the 49.9 ohm resistors (R26 and R27) shown in your schematic above, you should place a 4700 pF capacitor to ground from each unused VINQ pin. This allows the unused inputs to be properly self-biased by the internal Vcmo voltage connection.

Best regards,

Jim B