Confusion regarding Excitation Source in PXIe-4331 Bridge Input module

[Mirash]

Hello,

I got some confusion while going through the PXIe-4331 bridge input module Data Sheet.

In the Excitation Characteristics section, Its says Minimum Current as 29mA. What does this mean. It can delivery only current greater than 29mA!

I think this should me Maximum Current Specification.

If you look at the below table,

For 350 ohm resistance, the current should be 10V/350ohm = 28.5mA

but for 1K ohm, the current will me 10V/1Kohm = 10mA , which cannot be right if you specify minimum current as 29mA.

any thoughts?

Regards,

Mirash

[Brooks_C]

Hello Mirash,

I agree, this specification is somewhat ambiguous.

The excitation source on the PXIe-4330 and PXIe-4331 takes a voltage set point, but there is a limit to the amount of current it can provide.  If you specified 10 Volts of excitation and connected a 10 Ohm resistor between EX ± this would pull (V = I*R; 10V / 10 Ohms 😃 1 Amp of current.  Obviously the module cannot supply 1 Amp of current per channel.

What you would actually see in this case is that the channel would reach the current limit (which is specified to be at least 29 mA, and the actual limit may be a little bit more).  Basically the voltage would be reduced so that the current limit is not exceeded.  You would measure an excitation voltage of V = 29 mA * 10 Ohms = 0.29 Volts.  The specification is intended to be a minimum specification for the maximum current that the channel can supply.

In the case of the chart, think of that as the maximum voltage we can supply (that corresponds to one of the excitation values you can choose (0.625V, 1 V, 1.5V, 2.5V, 2.75V, 3.3V, 5V, 7.5V and 10V) without exceeding the current limit (which is 29 mA minimum). 

In the 120 Ohm full bridge case, at 3.3 V excitation you would get 3.3 V/120 Ohms =  27.5 mA.  However, at 5 V of excitation you would get 5 V / 120 Ohms = 41.6 mA.  We cannot supply 41.6 mA so you would hit the limit just above 29 mA and if you were to actually measure the voltage you would see 29 mA * 120 Ohms = 3.$8 V (or a little bit higher).

Let me know if this description helps and if not I can give it another go.

Cheers,

[Mirash]

Hello Brooks,

So if my Understanding is right, the Maximum current that can be provided by the the excitation source is 29mA.

The confusion lies in

---

  The specification is intended to be a minimum specification for the maximum current that the channel can supply.

---

When I looked in SCXI 1520 Specification, It is mentioned that Maximum Operating Current is 29mA. So this can be a Type O in 4331 data sheet.

---

In the 120 Ohm full bridge case, at 3.3 V excitation you would get 3.3 V/120 Ohms =  27.5 mA.  However, at 5 V of excitation you would get 5 V / 120 Ohms = 41.6 mA.  We cannot supply 41.6 mA so you would hit the limit just above 29 mA and if you were to actually measure the voltage you would see 29 mA * 120 Ohms = 3.$8 V (or a little bit higher).

---

Does this mean that the card cannot be used with 5V, 120 Ohm combination?

Regards,

Mirash

[Brooks_C]

Mirash,

I think our intent with the wording was to capture the fact that a channel may be able to provide slightly more than 29 mA.  However, I would agree that in nearly all cases "Maximum operating current" is much more clear.  I will be recommending the change to the specifications document.

Any preference between these two options:

"Maximum Operating Current......  29 mA"
"Maximum Operating Current......  29 mA minimum"

As for your question:

---

Does this mean that the card cannot be used with 5V, 120 Ohm combination?

---

In software you can specify whichever of the allowed excitation levels you want in combination with any bridge resistance you want and it will not cause an error.  So, if you specify 5V of excitation and connect a 120 Ohm load then the actual excitation voltage will be 29 mA * 120 Ohms = 3.48 V.

Best Regards,

[Mirash]

Hello Brooks,

Your explanation for the Current Specification is very Clear. Thank You!

I have one more doubt based on this query.

If I choose 5v, 120 Ohm combination and in actual it come as 3.48V..  Will it affect my measurement?  As I understand the bridge output voltage is directly propotional to the Excitation Voltage.

My Thanks and Regards,

Mirash

[Brooks_C · Accepted solution]

Hello Mirash,

I'm glad I could help!

As for your other question, the PXIe-4330/4331 are inherently ratiometric devices.  This means that they are designed to sense the excitation level and scale the ADC's range appropriately.  In this way we can continuously compensate for changes in the excitation voltage.  One of our engineers wrote a really nice Dveloper Zone article on the subject that I'd recommend you take a look at:  Measuring Bridge-Based Sensors with the Ratiometric Approach

Cheers,

[Mirash]

Thank you..!

The mentioned docuemnt clearly explains how the design removes the measurement dependence on the accuracy of the excitation voltage.

My Thanks and regards,

Mirash