Very low ref voltage

What kind of circuit topology are you thinking about ?

At least in any unbalanced circuits, the ground noise could swamp the reference voltage.

Most DC-accurate data acquisition boards have relatively high impedance inputs- gigohms, megohms.

Can't you just use a voltage divider off of an ordinary reference?

Il 21/06/2012 15:35, Spehro Pefhany ha scritto:

I'm sorry, I mean at F.S. And of course the requested current will be negligible in most applications.

In this case what kind of resistors should I use?

Thanks Marco

Ahh 1% metal film? Or the 0.1%ers if you want to be fancy. Getting low values (below 50 ohms) is expensive though. Do you really need a milli amp?

George H.

Il 21/06/2012 16:48, George Herold ha scritto:

The input stages are op-amps and INA as already pointed out by the other friend. Sensors have an impedance about 350 ohm typically (e.g. strain gauges).

If I use a 1.25V as standard ref, to have 100 uV I may use 1.5M over 120 ohm. Not bad.

Well, I think I over-estimated that :)

Marco

For 0.1%-ish lab stability, just about any precision metal film resistor will do.

Keep the lower end resistor to something like

Ref voltage followed by opamp to divide it? If necessary then followed by a length of constantan to divide it further, or even copper track. If R varies, Vout stays the same, only the impedance changes slightly

NT

"Here's a 1206 0.1% 10 ohm resistor +/-25ppm/=B0C for 44 cents."

"

Nice... ~$0.25 for 100. (I've been fixated on Susumu.)

George H.

Best (for accuracy) is to buy a divider or trimmed divider so the two resistors are the same material and batch. Look at DigiKey's 'RM32A' series voltage dividers, for instance.

Il 23/06/2012 22:54, whit3rd ha scritto:

Thanks for the hint. Marco

whit3rd schrieb:

Hello,

if the two resistors are on the same isolating base material, the thermal match is even better.

Bye

One way to get a stable and accurate voltage divider is to get a large number of XX kohm resistors, hopefully from the same batch.

To get a 10:1 voltage divider, put nine of these resistors in series and one to ground.

Likewise for current measurement, put multiple equal values in parallel and a single one through the actual current sensor.

This will compensate for the temperature coefficient as well as inaccurate XX values.

That's the way we do it in the integrated circuit world. Compensates out end-effects and TC.

I was able to make cheap 8-bit DAC's 50 years ago by applying that method.

...Jim Thompson

We had a similar problem and used the Nat Semi LM4140 as the ref, and the AD706 op-amp as the buffer. We have to be stable to better than 5V +/- 100 uV for hours, so the thermal drifts and 1/f noise didn't contaminate

Jon

A better way, in CMOS is to use gate leakage (tunneling current). The gate oxide thickness is held to very tight tolerances as are the gates themselves. It turns out to be a very good high value divider. Ratios other than 1/2 can be had by varying the gate size but it's not as good as a 1:1 divider. Of course you can't load it much, either. ;-)

Have used Mosfets for voltage references due to their T coefficiency being +, and - on the Vg(th).

For the better part, it seems to work. But, they don't seem to be very fast and you have to keep them with in a window. At least the Mfets I used were like that and they do operate in a cold environment.

I didn't use an RF mosfet and there was a reason for that, I just can't remember why?

Jamie

Creating too low a voltage reference introduces problems one must keep an eye out for.

Can't be considered 'stable' or even 'referenceable' once it gets below a certain point.

Pretty much why the industry has them where they are. They have already found the most stable, best value to provide.

Abating the 1/f drops the noise floor and get you a lot of benefit.

It is important to excise as much as possible.

Il 26/06/2012 05:16, SoothSayer ha scritto:

What is this "certain point", approximately?

Marco