Why would Analog Devices not suggest their lowest-offset opamps - Page 1
Forum section: electronics / projects

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 13, 2017, 11:34:52 am
content: Analog devices are currently selling one of the most linear DACs on the market, the AD5791. It really is quite the impressive part: 20 bit, 1ppm INL, 1ppm resolution, <0.05 ppm/degC tempco). When you do the math, this comes down to 10-20 uV precision and accuracy for a 10 V full scale.However, in all of their suggested circuits, they suggest using the AD8676 and AD8675 opamps. These too are quite impressive parts, being 12 uV offset and very low noise.However, Analog Devices have some parts with even lower offset, such as the ADA4522 series, which offers sub-microvolt offset (typical, 5 uV max). I don't understand why they would not suggest using these opamps for functions like buffering the reference into the DAC and such. The only "downsides" I can see when comparing the two is that the ADA4522 has lower GBW and more noise. However, since the reference voltage should be a DC value (by design), we don't need high speed and can use a lot of filtering to lower the noise.Is there any reason they would not use those parts spec-wise? I'm very interested in ultra-precision design but am still new to the trade so I would be curious to see if there are any "traps-for-young-players" that I might be overlooking here.Oh and links to the parts:DAC:http://www.analog.com/en/products/digital-to-analog-converters/ad5791.html#product-overviewAD8676:http://www.analog.com/media/en/technical-documentation/data-sheets/AD8676.pdfADA4522:http://www.analog.com/en/products/amplifiers/operational-amplifiers/zero-drift-amplifiers/ada4522-4.html#product-overview

author: RoGeorge
author_rank: Super Contributor
timestamp: December 13, 2017, 11:51:31 am
content: Sounds like a good question to ask Analog Devices about.At a draft look at the datasheets, ADA4522 are described as 'zero drift op amps' while AD8676 is not. 'Zero drift' usually means the op amp have an internal auto-adjustable compensation loop for auto-zero the offset. The control loop periodically measures the offset voltage then apply compensation internally.My guess is that the process of auto-zero offset (an op amp with variable parameters) will interfere with the DAC precision, so an op amp without auto-zero is preferred for it's constant parameters/performance, even if that constant performance comes with a little offset.

author: Echo88
author_rank: Frequent Contributor
timestamp: December 13, 2017, 12:09:09 pm
content: Most Auto-Zero-OP-Amps produce massive peak input current spikes due to the switching-nature of the architecture, this is not mentioned in the Datasheets made by the manufacturers. I would assume thats why they dont use those auto-zero-op-amps for reference buffering. Further info on the switching spikes can be found in the Art of Electronics 3, which i can only recommend.See also:http://www.analog.com/en/analog-dialogue/articles/20-bit-dac-and-accurate-precision-voltage-source.html"For the reference buffers, the AD8676 dual amplifier is recommended, based on its low noise, low offset error, low offset error drift, and low input bias currents. The input bias current specification of the reference buffers is important, as excessive bias currents will degrade the dc linearity."

author: Kleinstein
author_rank: Super Contributor
timestamp: December 13, 2017, 05:39:54 pm
content: The input current spikes can be a problem with AZ OPs. Also the ADA4522 is a rather new part - maybe after the DAC Circuit was designed. The noise (and spurs) spectrum of an AZ OP might not be that well suited.For the reference buffer, an offset will just add to the reference voltage - not many references are accurate to 12 µV from factory, maybe the 1 V range of a Fluke73x and similar.The reference is DC voltage, but when switching the DAC will need significant current spikes from the reference - so the reference amplifier must be fast to get reasonable fast settling. Otherwise DAC settling would suffer. Filtering of the reference can be a good idea, but still needs a fast driver toward the DAC.

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2017, 06:40:33 pm
content: I had to recreate and post this a second time because the gorram forum ate my first reply.Quote from: TheUnnamedNewbie on December 13, 2017, 11:34:52 amNoise from charge injection is a real problem with chopper stabilized amplifiers if they are not applied correctly. Their real advantage is at low bandwidths where they have lower 1/f noise than bipolar parts and this explains why they are not recommended here for general use. In this case, low bandwidth means below 0.8 Hz where the chopper ADA4522 has lower noise than the bipolar AD8675/AD8676. If the AD5791 DAC is being used in a sampling instead of integrating application, then the bipolar AD8675 is a better choice.There are ways to combine a bipolar amplifier with a chopper stabilized amplifier to get the best of both. Essentially the chopper stabilized amplifier is used to correct the offset voltage and offset voltage drift of the bipolar amplifier.