adjustable current sink in place of gain set resistor - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: August 05, 2014, 03:11:12 am
content: I have a DAC chip that uses a resistor ground referenced resistor to set full scale gain.Is there any reason why a digitally controlled adjustable current sink could not replace the resistor in order to create a full scale gain trim?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2014, 04:43:53 am
content: Do they give an internal schematic?

author: SArepairman
author_rank: Frequent Contributor
timestamp: August 05, 2014, 05:03:00 am
content: not really , I mean to disable the Rsetx resistor and replace it with a current sink. I find it kind of mysterious.I wanted more then 12 bits of gain adjustment while not getting the extra distortion penalty of using a MDAC attenuator/gain stage or the complication of a SC setup.I assumed they somehow set up something where gain is controlled by DC sink current. Or am I wrong and that resistor needs to be low inductance high frequency friendly? I am also worried about a minimum current requirement. I am not sure how big I could make that resistor and still have the chip work.I wanted finer tunability then what a digipot (internal one is only 4 bit, external low tempco one is like, 10 bit max) offers. I think a DAC controlled current sink could easily be 16 bits.Granted, 10 + 12 bits is probably enough, but I am still curious.

author: David Hess
author_rank: Super Contributor
timestamp: August 05, 2014, 06:31:15 am
content: A current will work fine but if the internal reference is used, then the voltage at the FSADJ pin is only going to nominally be 1.0 volts above ground. Many current output DACs can drive that directly but the current is only 250 microamps maximum and the AD9106 is only specified for full scale outputs of 2 to 8 milliamps which is only 2 bits of additional dynamic range.

author: SArepairman
author_rank: Frequent Contributor
timestamp: August 05, 2014, 06:55:31 am
content: Any clues as to what might happen if you go under 2mA output?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2014, 07:43:30 am
content: The diagram seems to indicate the resistors as being part of the operational amplifiers feeding the DACs. Most likely some signal voltage appears on those pins. What that signal is, I don't really know... are those just constant references?In any case, p.23 of the datasheet seems to be what you're missing..http://www.analog.com/static/imported-files/data_sheets/AD9106.pdfI wonder why you'd want to use constant current anyway (the internal cal is probably better than anything you'd do on the outside). If you must, I'd suggest asking an FAE rather than here.Tim

author: SArepairman
author_rank: Frequent Contributor
timestamp: August 05, 2014, 07:55:49 am
content: Quote from: T3sl4co1l on August 05, 2014, 07:43:30 amI wanted the constant current so I can have finer amplitude control. I could set the full scale to however many bits and then adjust that full scale by 12 bits, rather then having to settle for a single full scale (or a 4 bit full scale if I use the internal digitpot). I could set my full scale from 2-8mA at 370uA intervals with the built in digipot, but if I used something else that is more finely adjustable, I could adjust the full scale in tighter intervals.I missed the minimum current specification. It made more sense when I thought the DAC was capable of driving loads smaller then 2mA, I.E. use a 50K ohm load rather then their specified 16K max, allowing for a greater full scale adjust range). I figured it would probably be fineWhat could likely go wrong from increasing that RsetX value? snowballs chance in hell that it could still work? I see the obvious problem of trying to get more then 8mA out of it, but I don't see a obvious problem trying to get less then 2mA out of it. Why is there a minimum drive requirement?And I was going to use a V-I converter connected to a V out dac, but I guess sinking it right from a Iout DAC makes more sense.

author: SArepairman
author_rank: Frequent Contributor
timestamp: August 05, 2014, 10:42:43 am
content: in a previous thread of mine someone wrote"Some use the reference to drive transistors ( or FETs) switched inside the unit, and below the minimum level they do not turn fully on ( or off) enough to operate properly, giving rise to either a varying offset voltage and spurious outputs. To use a DAC with low voltage you need one with either a 4 quadrant reference input or one designed to be used as a variable attenuator.Otherwise use a level above the minimum and divide the output down to the desired level using a buffer opamp and a resistive divider, or a fractional gain opamp circuit."I guess its for similar reasons? Under 2mA get too low so things are running in the wrong regions? I guess I would need the transistor level diagram to understand where the 2mA guideline comes into play.

author: David Hess
author_rank: Super Contributor
timestamp: August 05, 2014, 11:18:42 am
content: Quote from: SArepairman on August 05, 2014, 06:55:31 amThe DAC performance will get worse in all respects.