How does digital to analog conversion in a DC standard work? (Knick)

[e61_phil]

Hello,

at work we have a 5.5 digit Knick DC calibrator. I wonder how the digital to analog conversion works. I can't find any DAC ICs. Is there a method like the dual slope method (A->D) in DMMs used in such devices which build a discrete digital to analog converter with great resolution?

Philipp

Edit: Sorry upside-down

[SeanB]

Probably a lot of R-2R BCD resistor networks on ceramic substrates that have been laser trimmed, along with a summing network to sum the discrete digits. You will likely see a lot of ceramic blocks with them inside under a shielding cover.

[e61_phil]

No, there aren't any ceramic resistor networks as far as I can see.

I can't believe a resistor network will work with 20bit resolution (5.5 digit = 1e6 counts ~ 20bit). The MSB accuracy have to be better than 1/(2^20)?

[SeanB]

I used to use some BCD ceramic networks that were made specially for the use. Came in a square ceramic block with 20 odd leads on the sides to give a voltage from 0 to Vref ( which was a 5 VAC 400Hz sine wave from the main AC reference generator) that was used to convert the input from a 5 digit BCD switch which had as the MSD a polarity switch. It was pretty stable, the LSD never drifted more than a count over the full operating range ( or from a bath in freezer followed by a cooking from a hot air gun) during testing. There was one trimming resistor for the unit, used to adjust the offset to zero. Typically I used a 1M resistor there to ground, as you needed a resistor in the schematic, but had a choice of connecting the chosen value to ref or GND.