I'm guessing the 4.5V offset will be used. I want to make a PC multimeter that uses Teraterm to display multiple voltages maybe 8. I need to scale down the measured voltages I want to know the best way of doing this. I did try a basic voltage divider with the output going into a buffer but there was some loading effects.

 

I did try a basic voltage divider with the output going into a buffer but there was some loading effects.

hi 2000,
Have you considered a buffer followed by the divider?
Use the non invert input of the OPA.
E
A post showing your actual circuit would help.

 

What are the source resistances of the voltages being measured ? I assume the basic input range of the ADC is 5 volts. Do all 8 voltages need scaling by the same amount ? Do they all have the same common reference ? Are they all positive or all negative of the reference point ?

Les.

 

Not sure what you mean by source resistances but the voltages could be measuring a 1k pot divider or a 10M pot divider with 10V input and giving a 5V output. I'll be using 5V for the input range with I think 2.5V being 0V and neg values will be from 0 to 2.5V and pos 2.5 to 5.

I think they will share the same REF+ and REF- but the VIN will be multiplexed through 8 channels.

Depending on the way the probes are positioned the measured value could be -5V or +5V so -5V will be represented somewhere between 0 and 2.5V will +5V will be represented somewhere between2.5V and 5V.

 

Assuming the source resistance of the 10 volts was zero ohms then the worst case with a 1 K potential divider woukd be when it consisted of two 500 ohm resistors. In that case it's output would have aa source impedance of 250 ohms. Similarly a 10 meg ohm potential divider would have a source resistance of 2.5 meg ohms. So it wold be like adding a 2.5 meg resistor in series with the top resistor of the potential divider that you use to scale the input range of your ADC.

Les.

 

hi,
Look at this option.
Note the input impedance.
Set the divider to suit your App.
E

 

thanks eric, I noticed a negative suppy was used. Does a multimeter use a negative voltage derived from the 9V battery? Do you think a 4.5V offset can used with a differential amp?

 

hi,
Its possible to have a CMV of say +4.5V with a IA amp powered by 9V supply.
Problem could be the Vout swing range is limited to approx +/-2.5V centred on +4.5V

I understand you want to measure Vin ranges of +/-10v, without loading the voltage point you are measuring.??
Using a 9v powered IA amp, will require some method of resistive dividing of the voltage being measured, before the IA amp input, which will means some loading of the measured point will occur.

E

 

Hi Eric, can you explain why you would have a +/- 2.5V swing and not a +/- 4.5V please and how this may be achieved if possible. Also you wouldn't have knowledge on how a commercial product such as Fluke would do this? I've tried googling it and can't find any explanation.

 

hi,
Most OPA' will not swing fully down to 0V or upto Vsupply.

If you use a Diff amp how do plan to to reduce the input signals from a +/-10v range, down to a value a 9v powered Diff amp would accept.?

Also as you plan to Analog select/ ADC the Vout voltages, will mean having a common ground/0v line, any +4.5v offset you apply to the Input signal, will appear as part of the signal you are measuring. [ ie: without isolation there will be a 4.5v level, relative to 0v, on the measuring probes]

E

 

I was going to scale them down first with pot divider. I was thinking Ref+ would be tied to the supply to the circuit and Ref- to gnd with Vin being the difference between the 2 probes. I was going to plot the straight line equation and have a formula to work out x for a given value of y.

It totally sure if this will work but I'm leaving as many options open as possible to get something working.

Can you confirm that a negative supply rail is not used in a standard multimeter?

 

Some DVM's do, ref this PDF. 7660 IC.
E

BTW:
Note the usage of high value resistors for dividing the measured input voltages.
If you considered using a OPA with a very input impedance eg: CA3240 you could copy the values from the printed Table.