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BUF634A: Increase slew rate of DAC7811 with BUF634a

user4858620
Prodigy 200 points
Part Number: BUF634A
Other Parts Discussed in Thread: OPA2277, , BUF634, THS3217, THS3215, OPA2810, OPA810

Hi.

I am using DAC7811EVM board.

I would like to increase the slew rate of DAC7811 output.

OPA 2277 on the evaluation board has a low slew rate.

So, I chose BUF634a with a high slew rate.

Which of the following two is the correct circuit configuration when combined with the evaluation board?
(Or are both configurations wrong?)
①.Swap OPA 2277 and BUF634a as shown below.
②.Add BUF634a after OPA2277 as shown below.
I thought that ② was limited by the SR of OPA2277, so the rise / fall time would not change even if BUF634a was added.
Therefore, I connected the DAC7811 evaluation board and the BUF634a evaluation board with jumper wires as shown in ① configuration.
But, no voltage was output.
Best Regard
  • 0
    Guru 48395 points

    Since the BUF634 is a unity gain buffer, it cannot increase slew rate, only output current drive  - its output just follows the OPA2277 output in the 2nd circuit above - so that OPA2277 slew rate is still what you get after the BUF634A

    Can you provide some targets to pick a device? In the 2nd circuit, keep in mind you are throwing away half your signal into ground and the external cap is setting the SSBW - that then gets into the available output transition rate. 

    You don't need this level of performance but a dedicated DAC output solution is delivered by the THS3215 and THS3217. Now there is some slew rate and full power BW. But again, with a desired Vpp output and slew rate, more progress can be made in a simple op amp solution. 

  • 0 in reply to Michael Steffes
    Guru 48395 points

    That relatively old DAC datasheet does list quite a few op amp options, there are some newer parts that might be of interest (OPA810/OPA2810) - but without output swing targets, cannot narrow down, 

  • 0 in reply to Michael Steffes
    Guru 48395 points

    Just scanning that table on the higher speed parts I am very familiar with, not even looking at the datasheets, this is  a riot of errors, be careful with that table, could seriously use some scrubbing. circled items are incorrect - and this is not even a thorough review, just obvious. 

  • 0
    TI__Genius 15555 points

    Hello,

    Were you able to find a suitable alternative for the OPA2277?

    Best,

    Hasan Babiker