DAC12DL3200: Biasing the outputs

Charan MMJ

Part Number: DAC12DL3200
Other Parts Discussed in Thread: TRF370417, LMH5401

Hi,

I am using DAC12DL3200 in my custom board. DAC is configured to generate signal in the frequency range of DC to several MHz. DAC outputs are given to a modulator which requires the common mode voltage of 1.7V. As per data sheet of DAC12DL3200 "The outputs VOUTx+/- must have a DC path to an external supply voltage, and the DAC sink current from the external supply".

I am not using any Balun or transformer in my circuit and want to use DC coupling between DAC and modulator as shown in "Figure 7-12. DAC Output Interface with DC Coupling" of the datasheet.

1. Shall I connect 1.7V as VBIAS?

2. What value of resistance should be used as RBIAS? (is it 3.6kohm)

3. Is the Common mode voltage at output equal to VBIAS?

Thanks in advance.

2 months ago

0 Rob Reeder 2 months ago

TI__Mastermind 23540 points

Hi Charan,

Vbias is 1.8V, not 1.7V. Not sure where you found that number.

Both of the differential outputs should be pulled to 1.8V.

It might be best to study the EVM design files as well.

Regards,

Rob

0 Charan MMJ 2 months ago in reply to Rob Reeder

Prodigy 70 points

Hi Rob,

Thank you for the reply.

The circuit I am interested in is the below circuit (DC-Coupling the DAC output to modulator).

As I want to connect this to the modulator (TRF370417) which needs a common mode voltage of 1.7V at the input lines, I wanted to ask if I can use 1.7V as VBIAS instead of 1.8V? (Please confirm if it can't be used as you mentioned)

Also, please confirm the value of RBIAS to be used?

0 Rob Reeder 2 months ago in reply to Charan MMJ

TI__Mastermind 23540 points

Hi Charan,

If you use 1.7V for Vbias, then the fullscale output will be different (lower) than expected. Also to DC couple to the modulator, you need to account for the input VCM and impedance as well.

Per the other post, it might be better to use a level shifter, LMH5401/3401 to do this.

Regards,

Rob

0 Charan MMJ 2 months ago in reply to Rob Reeder

Prodigy 70 points

Thank you Rob.

What value of RBIAS should be used?

0 Rob Reeder 2 months ago in reply to Charan MMJ

TI__Mastermind 23540 points

Hi Charan,

Its not that straight forward to recommend a specific value.

Since you will be DC coupling and using the LMH amp for the level shift you will need to simulate a value that will work first.

Then when you put the values on the board, you may need to iterate slightly.

Thx,

Rob

0 Rob Reeder 2 months ago in reply to Rob Reeder

TI__Mastermind 23540 points

Hi Charan,

See attached. This spreadsheet might help for your DC coupled interface.

Thanks,

Rob

DCCoupled_DAC-AMP_Calculator.xlsx

0 Charan MMJ 2 months ago in reply to Rob Reeder

Prodigy 70 points

Hi Rob,

Thank you very much

Data converters

Data converters forum

DAC12DL3200: Biasing the outputs