LM4128 2.5V reference problem 2

[mcgyvr]

Having a problem here..
I'm using the attached circuit to measure -75 to 75V and output it to 0-5V for sampling by a micro (2.5V -/+2.5V).
Except instead of the 2.5V voltage as shown in the image I'm using a LM4128 2.5V series voltage reference. (personally never used a voltage reference chip before)

Reading negative voltages works just fine and the output of the LM4128 stays stable at 2.5V. (Vout goes from roughly 0V to 2.5V as Vmeasured goes from -75 to 0V as I want)
The problem is measuring positive voltages.. At about 10V into Vmeasure I'm getting 4.5V or so at the output of the LM4128 causing the circuit to not work properly. (basically as Vmeasure is going up positive Vref is also going up)
??help

Here are the voltages I'm measuring at Vout (don't have the micro attached yet..All measurements made with a fluke multimeter)
Vmeasured----Vout
Negative input voltages
0V=2.5V
-10V=2.17V
-20V=1.84V
-30V=1.51V
-40V=1.19V
-50V=0.86V
-60V=0.53V
(basically .33V per 10V) OK..Seems to work fine..

NOW..
Positive input voltages
0V=2.5V
5V=2.75V
6V=3.18V
7V=3.64V
8V=4.02V
9V=4.57V
10V=4.57V (stops here and won't go any higher)

If I replace the LM4128 with a 2.5V power supply all works just fine.

 

[x7r]

I've never used this IC, but its datasheet implies that it is designed to work as a voltage source -- but not necessarily a load. So it supplies the intended load current polarity when your measured voltage is negative, but not when your measurement is positive.

 

[x7r]

Assuming my first answer is correct, I think a solution would be to add a resistor from the output of the LM4128 to a positive supply voltage (+12V, for example). The resistor value would be selected so that it drew roughly 2 mA of current from the regulator. Therefore the resistor value would be roughly 5kohm. That would keep the overall output current from the regulator positive.

 

[x7r]

Sorry -- I answered a phone call while I was typing the previous answer, and didn't immediately notice my mistake. I should have said that you can use a 5 kohm resistor to a negative supply, or possibly a 1 kohm resistor to ground.

 

[IRstuff]

A typical voltage reference cannot be used as a current sink. They are only designed for sourcing.

TTFN
faq731-376

 

[mcgyvr]

Thanks..
Off to find one that can source/sink. I wasn't aware of that limitation with voltage references.. Assumed they were just more stable/lower current voltage regulators.

 

[VEBill]

As hinted by xr7, you could possible add a load resistor to ensure that there's always a sink.

 

[mcgyvr]

xr7/ve1bill,
Simply adding a 1k resistor from the output of the LM4128 to ground solved the problem... Perfect..