OPA1632: design review please

Mark Thompson2

Part Number: OPA1632
Other Parts Discussed in Thread: OPA1637

Hi there,

I am hoping you could please review and make comment on the preliminary circuit / sketch of our proposed use of the the OPA1632 to provide a unity gain balanced signal line driver. The input signal is the balanced output from a DAC. I've asked a separate question in these forums for confirmation on how to power the OPA1632 from a single 12V DC supply, so this will be added to the circuit once I know.

The following is a snippet from our circuit. Note that I will have 24 differential DAC outputs to feed to 24 x OPA1632's to drive the balanced audio signal to external amplifiers.

The specification for our audio Codec suggests that at the output of each DAC that a single op amp, 3rd order, low pass filter is recommended. Is this something we can achieve with the following design using the OPA1632? Or would I need another op amp ahead of the OPA1632 to achieve this? We want to maintain the differential / balanced audio signal. Each output pin of the DAC has a nominal common-mode DC level of 1.5V and swings +/- 1.27 V for a 0 dBFS digital input signal. Again, will the following design with the OPA1632 accommodate this?

Thank you.

Mark

over 2 years ago

0 SimaJalaleddine over 2 years ago

TI__Mastermind 22250 points

Hello Mark,

Is this a voltage or a current output DAC? If current output, you would need a bias network or circuit prior to the FDA. What is the bandwidth requirement for the 3rd order low pass filter?

Thank you,
Sima

0 Mark Thompson2 over 2 years ago in reply to SimaJalaleddine

Intellectual 610 points

Hi Sima,

Thanks for your quick response! It's a voltage output DAC. All I really want from the LPF is that we have a flat passband up to fs/2 = 24000 Hz. It is for audio signals.

Kind regards,

Mark

+1 SimaJalaleddine over 2 years ago in reply to Mark Thompson2

TI__Mastermind 22250 points

Hello Mark,

Thanks for the additional information! Oh that is right, I should have assumed Hz to 20ishkHz for audio application. Then, yes, the OPA1632 will be able to achieve this design. Another option is the OPA1637, the datasheet for this device has a in-depth explanation on interfacing DAC with a 40kHz Butterworth response.

For a flat passband, I would recommended using a Butterworth response multiple feedback topology. A sallen-key topology is not possible with a fully differential amplifier.

Some references below:

1. Active Filtering Circuit for Audio DACs

2. DAC Post Filtering Circuit

3. MFB Filter FDAs (page 33)

4. Filter Design Program

Thank you,

Sima

Audio

Audio forum

OPA1632: design review please