Using an OP amp as output buffer of a DAC - Page 1
Forum section: electronics / projects

author: OM222O
author_rank: Frequent Contributor
timestamp: October 10, 2018, 06:03:12 pm
content: Hello, I'm building an electronic load and I'm using the 12 bit DAC6551 and a 4.096 voltage reference, to set the voltage refrence for an op amp which basically controls a mosfet gate voltage (via a 100resistor) according to the voltage drop across the ballast resistor. The DAC seems to have an integrated op amp on it's output so I was wondering if I need another buffer op amp as I've seen many people use it. I really need a rail to rail output so I think I should use a split supply for the external op amp if I use one. I'm also considering using a 128 bit 5k digital potentiometer to calibrate out any errors and possibly have a x2 gain functionality as well.I would really appreciate any feedback as if it is possible to just use the internal op amp of the DAC to get the desired functionality as it saves me quiet a bit on buying an external op amp.

author: _Wim_
author_rank: Super Contributor
timestamp: October 10, 2018, 07:35:20 pm
content: Quote from: OM222O on October 10, 2018, 06:03:12 pmAccording to the datasheet the internal opamp is specified as this:Output AmplifierThe output buffer amplifier is capable of generating rail-to-rail voltages on its output, giving an output range of0 V to VDD. It is capable of driving a load of 2 kΩ in parallel with 1000 pF to GND. The source and sinkcapabilities of the output amplifier can be seen in the Typical Characteristics. The slew rate is 1.4 V/μs with a fullscale setting time of 8 μs with the output unloaded.If you do not exceed this loading, no addtional opamp is needed.Quote from: OM222O on October 10, 2018, 06:03:12 pmWhen using a single supply rail to rail opamp, a split supply can be avoided in this application.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 10, 2018, 08:07:42 pm
content: You should specify the slew rate you're aiming for for your dac-controlled load.Unless you require very fast rise and fall times, the integrated opamp is more than enough. The series resistor to the gate will limit the current while charging the gate's capacitance. I would increase the resistor's value up to 1k though depending on the MOSFET you use.

author: rstofer
author_rank: Super Contributor
timestamp: October 10, 2018, 10:42:27 pm
content: I have always thought that "rail-to-rail output" is marketing. How in the world can the output driver not have at least a little voltage drop when loaded - even with just 2k?I would always try to design away from the rails. I guess it depends on the DAC.Having said that, using split rails guarantees you can get to true 0V on the output if you get to 0V on the input or if you calibrate out any offset caused by the DAC.I just don't trust working in the corners. To be fair, I haven't tested any single rail op amps working at the rails nor have I spent a lot of time scouring datasheets to see what the devices will really do.

author: coppercone2
author_rank: Super Contributor
timestamp: October 10, 2018, 11:55:23 pm
content: i need to read more but a rail to rail transistor level topology explanation would be interesting to see

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2018, 04:43:19 am
content: The DAC is fully buffered so there is no reason to use an external buffer unless greater capability is required.Quote from: rstofer on October 10, 2018, 10:42:27 pmThat is a very real limitation of "rail-to-rail" outputs. They do not really operate rail-to-rail but for many applications they get close enough. When the output transistors operate in saturation, the output resistance without feedback increases to 10s or 100s of ohms.

author: OM222O
author_rank: Frequent Contributor
timestamp: October 11, 2018, 11:48:48 am
content: Well I do need the absolute 0 volt (or even negative) output to propetly turn off the output fet, otherwise I would have a few mA at the output no matter what. The high rail won't be a problem as it's powered from 5V but 4.096 is the maximum output.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 11, 2018, 02:40:23 pm
content: Quote from: David Hess on October 11, 2018, 04:43:19 amYes. Another thing that should be considered is R-R inputs vs R-R output. On some R-R opamps, their operating range on inputs may be narrower than their output voltage span.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 11, 2018, 02:56:39 pm
content: As far as I've seen, this DAC guarantees a true 0V output and in case the 0V setting still had too high an offset, you can apparently put this DAC in shutdown mode and have its output pulled-down to GND. So that should allow a complete shutdown of the MOSFET.

author: _Wim_
author_rank: Super Contributor
timestamp: October 11, 2018, 07:04:41 pm
content: Quote from: OM222O on October 11, 2018, 11:48:48 amYou could use an AD822 for this, this can go below ground using only a single rail:http://www.analog.com/media/en/technical-documentation/data-sheets/ad822.pdfFrom the datasheet:The AD822 is a dual precision, low power FET input op amp that can operate from a single supply of 5 V to 30 V or from dual supplies of ±2.5 V to ±15 V. It has true single-supply capability with an input voltage range extending below the negative rail, allowing the AD822 to accommodate input signals below ground while in the single-supply mode. Output voltage swing extends to within 10 mV of each rail, providing the maximum output dynamic range.