Higher current DAC buffer

I need an op amp with relatively high current capability (up to 30-40 mA) to be used as DAC buffer.Output voltage range:0-5V. I would like to avoid the need of negative supply voltage, so it must be be RRO and RRI to Vs-. In addition it must be either RRI to Vs+ or allow Vs+ >5V. For all practical purposes it is a DC application, so dynamic characteristics like bandwidth, slew rate etc do not matter. On the other hand it must be as low noise, and precise (low Vos) as possible.

I found ADA4897 to be the best match, but with so much models it is easy to miss something.Are there other alternatives that meet requirements?

Now, I also would like to know, what could I gain if I gave up RR requirements. So, are there alternatives, that are not RR, but otherwise offer significant advantages over ADA4897 (and possible alternatives to it)?

I also would like to ask questions specific to ADA4897.

How large are voltage swings to supply rails at 0, +-10, +-20, +-30, +-40 mA?

Is it possible to calculate power dissipation with known output voltage and current? How?

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  • 0
    •  Analog Employees 
    on Oct 18, 2016 6:21 AM

    Hi Andrey, 

    You could try checking out the AD8655. It has low offset - 50uV typical and 250uV max, and low noise so it would be good for a DC application. It is RRIO (rail to rail in and out) and has a high current output capability. A plot from the datasheet is below, it should give you an idea of the typical headroom you would need from the rails given your current load:

    This plot shows that you would got to about 300mV from the rails if you went up to 40mA of load current. 

    One problem with the AD8655 though is that because of the architecture required to achieve rail to rail outputs, you would get some distortion somewhere close to the higher rail. This would demonstrate itself in a change in offset voltage as the input common mode voltage range approached 5V in your case. It is a fairly large change so you would need to account for this to assure linearity in your measurements. What is the resolution of the DAC that you will be using? 

    I will ask around and see whether I can get this kind of information for the ADA4897. One thing to note though is that the ADA4897 does not have a rail to rail input, only its output is rail to rail so you would be losing some range at the input. Also, with a large bandwidth, you would tend to get more noise in your circuit so if you decide to use the ADA4897, you might want to filter that out. 

    As for your question on power, the power can be calculated by the formula, 

    PD = (ISY × VSY) + ILOAD × (VSY –VOUT)

    You can check MS-2251 for more information on that. 

    Give the AD8655 a look and let me know whether it works for you.

    Thanks, 

    Kris

Reply
  • 0
    •  Analog Employees 
    on Oct 18, 2016 6:21 AM

    Hi Andrey, 

    You could try checking out the AD8655. It has low offset - 50uV typical and 250uV max, and low noise so it would be good for a DC application. It is RRIO (rail to rail in and out) and has a high current output capability. A plot from the datasheet is below, it should give you an idea of the typical headroom you would need from the rails given your current load:

    This plot shows that you would got to about 300mV from the rails if you went up to 40mA of load current. 

    One problem with the AD8655 though is that because of the architecture required to achieve rail to rail outputs, you would get some distortion somewhere close to the higher rail. This would demonstrate itself in a change in offset voltage as the input common mode voltage range approached 5V in your case. It is a fairly large change so you would need to account for this to assure linearity in your measurements. What is the resolution of the DAC that you will be using? 

    I will ask around and see whether I can get this kind of information for the ADA4897. One thing to note though is that the ADA4897 does not have a rail to rail input, only its output is rail to rail so you would be losing some range at the input. Also, with a large bandwidth, you would tend to get more noise in your circuit so if you decide to use the ADA4897, you might want to filter that out. 

    As for your question on power, the power can be calculated by the formula, 

    PD = (ISY × VSY) + ILOAD × (VSY –VOUT)

    You can check MS-2251 for more information on that. 

    Give the AD8655 a look and let me know whether it works for you.

    Thanks, 

    Kris

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