Instead of manually adjusting a pot (voltage divider) to set the
desired current level through a servo loop (op-amp driving a FET), I'd
like to use a PIC controlling a DAC. My problem is that even
rail-to-rail DACs only go down to a few millivolts, not true zero
volts, when the DAC code is zero. This few millivolts prevents me
from setting my current levels low enough for my use. I have a bunch
of servo loops and it adds up quickly.

I was thinking that I could have the PIC multiply the desired DAC
output code by X and then set up a resistor voltage-divider on the
output of the DAC to divide the voltage by that same X amount (before
going on to the servo loop's op-amp). This would reduce the zero-code
output voltage level by X times and give me a much lower minimum
current level for each servo loop. Using my existing setup, I can set
X = 30 or so, maximum.

Is this a reasonable way to reduce the effect of the DAC output not
being able to reach a true zero volts? Is there a better way? Level
shifting of some kind?

Thanks!
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Instead of manually adjusting a pot (voltage divider) to set the
desired current level through a servo loop (op-amp driving a FET), I'd
like to use a PIC controlling a DAC. My problem is that even
rail-to-rail DACs only go down to a few millivolts, not true zero
volts, when the DAC code is zero. This few millivolts prevents me
from setting my current levels low enough for my use. I have a bunch
of servo loops and it adds up quickly.

I was thinking that I could have the PIC multiply the desired DAC
output code by X and then set up a resistor voltage-divider on the
output of the DAC to divide the voltage by that same X amount (before
going on to the servo loop's op-amp). This would reduce the zero-code
output voltage level by X times and give me a much lower minimum
current level for each servo loop. Using my existing setup, I can set
X = 30 or so, maximum.

Is this a reasonable way to reduce the effect of the DAC output not
being able to reach a true zero volts? Is there a better way? Level
shifting of some kind?

Thanks!
-- remove SPAMMENOT for e-mail responses --

Instead of manually adjusting a pot (voltage divider) to set the
desired current level through a servo loop (op-amp driving a FET), I'd
like to use a PIC controlling a DAC. My problem is that even
rail-to-rail DACs only go down to a few millivolts, not true zero
volts, when the DAC code is zero. This few millivolts prevents me
from setting my current levels low enough for my use. I have a bunch
of servo loops and it adds up quickly.

I was thinking that I could have the PIC multiply the desired DAC
output code by X and then set up a resistor voltage-divider on the
output of the DAC to divide the voltage by that same X amount (before
going on to the servo loop's op-amp). This would reduce the zero-code
output voltage level by X times and give me a much lower minimum
current level for each servo loop. Using my existing setup, I can set
X = 30 or so, maximum.

Is this a reasonable way to reduce the effect of the DAC output not
being able to reach a true zero volts? Is there a better way? Level
shifting of some kind?

Thanks!
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you need some small amount of - Voltage.

Yea, I shold have posted the circuit. It's Win Hill's recommended
circuit from a while ago and has worked well when manually adjusting
the pot off the reference.

I'm using a .005-ohm shunt so if the DAC (MCP4921) can only drop to
10mV with a DAC code of zero (instead of 0.000V) then each FET will
set up to pass (0.010mV/0.005 ohms) = 2 amps per FET as the minimum
current level I can set. Too high. I can't increase the resistance
of the shunt anymore (current level is too high) so I was thinking I
could drop the DAC's min. output voltage with a voltage divider.

Using my example of divide-by-30 resistors on the output of the DAC,
the 10mV output for a DAC code of zero would drop to 0.33mV. This
would give me a min. current level of 67mA through each FET. Not
ideal, but I could live with that.

I'll look over the other ideas brought up in other posts. I'm sure
I'll have a couple more questions though. :-)





SHUNT = 0.005 ohm, 1%, 5W
R6 = 1K, 1%
. Vcc R2, R3 = 49.9K, 1%
. | R4, R5 = 10K, 1%
. R1
. |
. +------, Vcc ,-----+-----
. | | | LT1013 | x |
. 1V REF POT <-+--R2--+-----|+\ D |
. | | | | | >--+--R6-- G |
. -+------+-, | | ,--|-/ | S |
. | | | | | === C1 | |
. low-level | | | | | | | |
. signal GND +--|--R3--|--+----|---+---R5----+ |
. | | | | | |
. | | | | SHUNT |
. | | | | | y |
. | | '--R4---+-------------+-----|-+--- RTN
. | | | |
. | | | |
. | | Vcc ,-----+ |
. | | | | | |
. | +--R2--+-----|+\ D | |
. | | | | >--+--R6-- G | |
. | | | ,--|-/ | S | |
. | | | | | === C1 | | |
. | | | | | | | | |
. GND +--|--R3--|--+----|---+---R5----+ | |
. | | | | | | |
. | | | | SHUNT | |
. | | | | | | |
. | | '--R4---+-------------+-----|-+
. | | | |
. : : etc etc - N stages : :
. : : : :
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you need to use a + - rail supply so that you can balance

You may find that the easiest and most compact solution you can do is
to supply a negative rail so you _can_ bring your voltages down to some
true zero.

An alternative is to supply yourself with a virtual ground.