DAC zero code = zero volts, best way??

Forgot to mention, I have a +5V supply to the PIC and DAC and a +1V reference to the DAC. No negative voltages available.

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Hmm. Why not post a schematic on a.b.s.e?

Cheers,

Phil Hobbs

I can't tell if your problem is that you can't hit zero or that you just can't get *close enough* to zero. In other words, is the code '1' too high (or too coarse) or is '0' too high for '0'.

If you want to scale the output ('0' still not 0V, but '1' lower and each step finer) then an output divider is fine. However, mind the input impedance of your next stage. If you are relying on a buffer built into the DAC to drive your next stage, the relatively high output impedance resistor divider is going to need another buffer. That buffer, even if *IT* is rail-to-rail is going to cause you more offset/scale problems near 0 again.

You mention in your add-on that you have no negative voltages available. 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. Normally I'd choose a voltage that's 1/2 my supply voltage (less if I'm using el cheapo op amps like the LM358 that go close to ground but only approach the top rail to within 2V or so). It sounds like you don't need to servo on the negative side, so you may be able to do OK with a 0.5V, or some other low value which you know your DACs can always fall below. If you need a really precise zero you'll have to develop this voltage carefully.

If you don't need an exact amount of shift you can drive a resistor through a diode. This will always put you one diode drop below the DAC output, but you will learn amazing things about how much a diode drop can vary with current and temperature. This technique also won't work if your following stages are dumping current into the resistor.

"John" schreef in bericht news: snipped-for-privacy@4ax.com...

Why not make a small negative voltage? Have one of the PIC outputs generate a square wave, couple the output with a nice cap and follow with a rectifier, regulator etc.

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

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 --+--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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I vote with this - that's what I'd do if I needed a "real" zero. Admittedly, the last time I did this I already had the negative supply, but as Frank writes, enough negative volts for bias should only cost a few parts.

Good Luck! Rich

"A rectifier"?

Two caps, two diodes:

|| o----||--o--->|---. || | | | V | '---|

you need some small amount of - Voltage.

you need to use a + - rail supply so that you can balance the op-amp at zero.

Oh sure, that must be a language problem. Rectifier and diode are the same thing for me. I certainly didn't mean a bridge-rectifier. There is a nice improvement half way down this page:

I'm beginning to think that may be the case. The neg voltage ripple might be a problem though but testing will tell.

Yea, might get ugly quick though. A quick&dirty true neg. voltage or fully regulated one (if needed) seems to be my best bet so far.

Thanks!

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Thanks for the recommendation and the ckt! I'll check some specs and whip up a prototype to test.

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If you use a charge pump at -5V (you have +5V available) and then regulate to -3.3V (or some other arbitrary value more suitable in your view, but at least 1.5V away from the rail) you should be able to keep the negative rail ripple quite manageable.

I have found the charge pumps work perfectly well from -Vin to -Vin + 1 across the load range of perhaps 100mA. With a followon LDO, that should work perfectly well.

Cheers

PeteS

I'd just tie one of the unused I/O pins to the DAC output via a resistor. I think you can set the I/O pin to high impedence. Then, when you want to go to 0 volts (ground,) just let the second I/O pin be a digital output and set it to "0" and draw the voltage to ground (sink the current.)

Almo

PeteS wrote:

"almo" schreef in bericht news: snipped-for-privacy@b28g2000cwb.googlegroups.com...

First of all you are a stupid top poster, and secondly the OP wants to get rid of the last few mV so your idea is totally worthless.

PLONK and good riddance.

Hey...I'm only in 7th grade. Cut me some slack.