NI 9209 - Resolution of Voltage Measurement

[linu95k]

Hi,

I am trying to understand the smallest measurable voltage change by an analog input module.

For Example, the NI 9209 has an input range of +/-10V, and 24-bit ADC so:

1. 10/(2^24) = 0.00000005V

A change of minimum 0.0000005V can be measured?

or is it

2. -10 to +10 = 20V scale, thus 20/(2^24) = 0.000001V

A change of minimum 0.000001V can be measured?

Can I use a Voltage divider logic to measure the resistance of a copper wire accurately?

1. Two Resistor R1 (220Ohm), R2 (Wire for which the resistance is to be measured) are connected in series with Each Other

2. One End of the 220 Ohm resistance is connected to AO+ and AI1+

3. The common terminal between R1 & R2 are connected between AI1- and AI2+

4. The other terminal of R2 (Wire) is connected between AO- and AI2-

Assuming,

The output of AO is 5V, we can measure the voltage drop across 220Ohm and find the current in the loop

Current I = V1(measured)/220 (Known resistance)

then,

The Resistance of the wire is,

R2(Wire) = [V1/V2] * 220

where V1 is measured voltage drop across R1

and V2 is the measured voltage drop across R2

Am I on the right Path?

[GerdW]

Hi linu,

---

@linu95k wrote:

I am trying to understand the smallest measurable voltage change by an analog input module.

For Example, the NI 9209 has an input range of +/-10V, and 24-bit ADC so:

1. 10/(2^24) = 0.00000005V

A change of minimum 0.0000005V can be measured?

or is it

2. -10 to +10 = 20V scale, thus 20/(2^24) = 0.000001V

A change of minimum 0.000001V can be measured?

---

Simple calculation: When the AI can measure a range of 20V (±10V) then a 24bit ADC has a resolution of 20V/2^24 = 1.1921µV.

Btw. using SI prefixes (like µ) greatly improves readability of very small or large numbers…

---

@linu95k wrote:

Can I use a Voltage divider logic to measure the resistance of a copper wire … ?

---

Yes.

Simple voltage divider, simple Ohm's law, simple Kirchhoff's rules…

(Basic knowledge for any electrical engineer or even student!)

---

@linu95k wrote:

Can I use a Voltage divider logic to measure the resistance of a copper wire accurately?

---

No.

To measure accurately you need to use special resistors, keep them at a defined temperature, avoid any thermocouple effects, keep the DUT under defined conditions, adapt the measurement setup to the expected resistance, …

Basic knowledge for any electrical engineer or even student!

[linu95k]

The caliberation logic can be done in software right?

using Actual values = Offset + Gain * Measured value

[AndersSekanina]

Hi linu95k

You are right when it is a +-10V, then the total scale is 20V, thereby 20V/2^24 resolution. Or a little less, because the maximal input range is 10,4V. But you can´t expect to use the lowest bits, because they will be hidden in the noise. 

Yes you can use that principle to measure the resistance of a wire, but if the resistance of the wire is much less than 220R, it can be difficult to get a good result, even with a 24bit ADC. But test it, and if the results are not as expected, write about that.

Regards

Anders Sekanina

[GerdW]

Hi linu,

---

@linu95k wrote:

The caliberation logic can be done in software right?

using Actual values = Offset + Gain * Measured value

---

Where does "calibration" come into play with your initial question?

That equation (y = a*x+b) only describes "scaling". "Calibration" is soemthing completely different…

[linu95k]

Is there a better way to measure resistance using NI Modules?

We could use NI 9218 or NI 9237, however we need 56 simultaneous channels.

What I mean is I want to perform resistance measurement for 56 cables simultaneously.

[santo_13]

---

@linu95k wrote:

Is there a better way to measure resistance using NI Modules?

We could use NI 9218 or NI 9237, however we need 56 simultaneous channels.

What I mean is I want to perform resistance measurement for 56 cables simultaneously.

---

The best instrument to measure resistance is a DMM or Ohm meter as those are calibrated.

When you say 'simultaneously' do you have enough budget to buy 56 DMMs if needed ?

[AndersSekanina]

If you are looking at DMMs, you should have a look at Keysight DAQ970A with 3 DAQM901A. There you have 60 "simulations" (multiplexed) current measurements, with a accuracy of some mOhm. There is a LabVIEW driver for it.

I have used that model in many test setups.

[linu95k]

Thank you for your precious feedback but I need to conduct 56-Simultaneous measurements so I don't think a multiplexed module Multimeter would help. 

[RIObotics]

NI 9219 is also an adequate module for accurate resistance measurements.  You may want to look into it, as it offers 4-wire resistance measurements.

To expand your cDAQ system for 56 simultaneous channels, you can also consider daisy-chaining multiple cDAQ-9189 8-slot TSN chassis together.  These cDAQ units have built-in timing and synchronization across TSN ethernet, I've used these before, it's quick and easy to setup.

BR

John

Add motion to LabVIEW in 30min or less - TENET EMotion
Finding it hard to source NI hardware? Try NI Trading Post