Why Hp Using Audio DAC in E3631 PSU?

[nour]

I was reviewing E3631 schematics and while I was viewing the DAC section, I found that they are using an audio grade DAC --> AD1851S!
I also saw the same DAC in other models!

Why they have used this audio grade DAC in a PSU?
Shouldn't an audio grade DAC be less accurate when it comes to DC output accuracy? please correct me if I am wrong!

I am asking because I am trying to make My own PSU and decided to use ltc2642 16bit DAC based on some parameters in the ltc2642 datasheet and when I saw this one in their design I was a little bit confused, I didn't use Dac before so I don't have practical experience with them to make a good comparison !

[Kleinstein]

Audio DACs usually have a relatively high gain drift and are thus not the best choice for high accuracy applications. However they are rather cheap and thus may be an alternative if highest accuracy is not needed. So you may get a 16 bit Audio DAC for less than a 12 Bit precision DAC. Depending on the requirements the audio DAC might be the better choice than a high grade 12 bit DAC.

The AD1851 seem to be not that bad with a 25 ppm/K gain drift and rather low offset drift.
Over a +-10 K temperature range this would be about +-1 LSB for 12 Bit DAC. For a power supply this can be well good enough.

An other point might be, if they use a good ADC to do an gain correction. So the DAC would be only for resolution, that accuracy would be from the ADC. Sometimes a good ADC is cheaper than a good DAC and they need a ADC for the display anyway.

[fonograph]

what is offest drift?

[nour]

Audio DACs usually have a relatively high gain drift and are thus not the best choice for high accuracy applications. However they are rather cheap and thus may be an alternative if highest accuracy is not needed. So you may get a 16 bit Audio DAC for less than a 12 Bit precision DAC. Depending on the requirements the audio DAC might be the better choice than a high grade 12 bit DAC.

The AD1851 seem to be not that bad with a 25 ppm/K gain drift and rather low offset drift.
Over a +-10 K temperature range this would be about +-1 LSB for 12 Bit DAC. For a power supply this can be well good enough.

An other point might be, if they use a good ADC to do an gain correction. So the DAC would be only for resolution, that accuracy would be from the ADC. Sometimes a good ADC is cheaper than a good DAC and they need a ADC for the display anyway.

There something that doesn't adds up in your comment to me

However they are rather cheap and thus may be an alternative if highest accuracy is not needed

Isn't that PSU over 1400$ new!!!!!
why would they use a cheap DAC in such an expensive instrument?
I know there are still other good and reliable functionality in the unit, but it is very expensive actually compare to its output power\resulotion and accuracy!
It is more expensive than a modern 6.5 digit  Keysight sold new!! and HP call this unit a basic power supply 

what is offest drift?

It is the drift that happens over temperature usually or over time to the offset input of and amplifier\instrument

[Kleinstein]

The drift in die Audio DACs is to a large part over temperature. There might be a little aging too.

Not all choices the engineers at HP / Keysight do are good. So there might be a few bad ones too. With a more lower quantity product the price of the components is sometimes not that important - if it works they usually don't change it, if there is no absolute need to.

[Jay_Diddy_B]

Hi,

I had a quick look at the E3631 schematic. The ADC circuitry is much better than the DAC and the rest of the power supply from a precision point of view. For example the TL074 op-amps used in the output voltage control have a typical offset drift specification of 18uV/C. This is gained up by the circuit, around 12, to 216uV/C.

Also the DAC is shared, using a mux and track and hold circuits between 8 outputs. (Tektronix did this in the TDS5xx and TDS7xx scopes. It is one of the things that goes bad if the electrolytics leak on the board)



The ADC is much better. It is *possible*, I don't know, it depends on the firmware, that the DAC is being disciplined by the ADC, so that it doesn't need to be that good, just have lots of resolution.

Regards,

Jay_Diddy_B

[Fungus]

Maybe they're cheaper. 

It doesn't matter so long as they meet the spec.

[nour]

This part is up to almost 15$ on Digikey for 1 part and almost 11.5 per quantities
https://www.digikey.com/product-detail/en/analog-devices-inc/AD1851RZ/AD1851RZ-ND/936661

It is not cheap now and I don't think it was 20 years ago!

[Kleinstein]

Yes the AD1851 is not a cheap audio DAC. However 20 Year ago, precision DAC with high resolution where even more expensive. So today it would not be a good solution, and it might not have been the very best choice back than.