# Best Way to Implement Full Bridge Balance with AD623

#### vane

Joined Feb 28, 2007
189
Hi guys/gals,

I've got a 0 - 15PSIA 4 wire full bridge transducer (annoyingly, it's old and discontinued so no datasheet).

When exciting with 5v and 0v at ambient pressure it has an output voltage of 1700mV and the deepest vacuum I can draw at the moment 1692mV read on a cheap Uni-T multimeter.

Do any of you with precision measurement experience know what the most precise way of balancing this is? Should I put a high precision pot in parallel across two of the transducer wires to balance it? This might cause a loss of any linearity on the sensor so I'm not sure this is the best idea.

Current circuit is:
Transducer:
Red - +vs 5v
Black - Gnd
Green - +
White - -

REF of AD623 is connected to a 10k POT between Gnd and +vs, this was my attempt at adjusting the offset of the reading.

#### ericgibbs

Joined Jan 29, 2010
11,548
REF of AD623 is connected to a 10k POT between Gnd and +vs, this was my attempt at adjusting the offset of the reading
hi vane,
The Vref must be powered from a low impedance source, a pot will create linearity problems.

How far off balance is the bridge output.? mV.??
E

#### vane

Joined Feb 28, 2007
189
hi vane,
The Vref must be powered from a low impedance source, a pot will create linearity problems.

How far off balance is the bridge output.? mV.??
E
Do you mean the REF pin or the excitation voltage?

The impedance of excitation wires at ambient pressure is 720 ohms and output wires is 746 ohms. The bridge outputs 1700mV at rest.

#### OBW0549

Joined Mar 2, 2015
3,566
I've got a 0 - 15PSIA 4 wire full bridge transducer (annoyingly, it's old and discontinued so no datasheet). When exciting with 5v and 0v at ambient pressure it has an output voltage of 1700mV and the deepest vacuum I can draw at the moment 1692mV read on a cheap Uni-T multimeter.
That is an awfully large output for a raw (i.e., un-amplified) bridge transducer; the MPX200 sensors in my hobby stock are spec'd at 60 mV typical full-scale output. 1700 mV is just way out of line, to the point of seeming impossible. Are you absolutely, positively sure the sensor isn't defective/broken?

REF of AD623 is connected to a 10k POT between Gnd and +vs, this was my attempt at adjusting the offset of the reading.
Connecting the REF input of an instrumentation amp to a resistive source messes up the common-mode rejection of the device. Far better to buffer the arm of the pot with a unity-gain op amp, and feed the REF input from the op amp, so the REF input sees a zero ohm source impedance.

Do any of you with precision measurement experience know what the most precise way of balancing this is?
The times I've had to do this, I've connected a trimpot between +Vs and GND, with the arm of the pot connected through a large-ish resistor to one of the two bridge output signals. Make the resistor small enough to get the adjustment range you need, but no smaller; otherwise you'll alter the bridge scale factor.

#### vane

Joined Feb 28, 2007
189
That is an awfully large output for a raw (i.e., un-amplified) bridge transducer; the MPX200 sensors in my hobby stock are spec'd at 60 mV typical full-scale output. 1700 mV is just way out of line, to the point of seeming impossible. Are you absolutely, positively sure the sensor isn't defective/broken?

Connecting the REF input of an instrumentation amp to a resistive source messes up the common-mode rejection of the device. Far better to buffer the arm of the pot with a unity-gain op amp, and feed the REF input from the op amp, so the REF input sees a zero ohm source impedance.

The times I've had to do this, I've connected a trimpot between +Vs and GND, with the arm of the pot connected through a large-ish resistor to one of the two bridge output signals. Make the resistor small enough to get the adjustment range you need, but no smaller; otherwise you'll alter the bridge scale factor.
The person I got the sensor from said he managed to get about a 30mV range from the 0 - 15PSIA with a -5v +5v excitation. It did seem a bit high on the output side, my past experience of stick on strain gauges were they were pretty well balanced. I used to use a Vishay P3 gauge reader for work, and for that you just pressed a button to balance it.

I was hoping to get the simplest circuit possible, can REF be connected to ground without incident for now then?

Will your balance work around greatly affect the precision of the reading trying to balance out 1700mV of imbalance? I realised you are at the mercy of thermal conductivity coefficients possibly interfering with readings.

#### ericgibbs

Joined Jan 29, 2010
11,548
If you are using dual supplies for the bridge and AD623, Vref can be 0V.

A small offset voltage can be applied using a high value resistor to one of the inputs of the AD623, add a trim pot for fine zeroing.
E

#### OBW0549

Joined Mar 2, 2015
3,566
I was hoping to get the simplest circuit possible, can REF be connected to ground without incident for now then?
Yup.

Will your balance work around greatly affect the precision of the reading trying to balance out 1700mV of imbalance? I realised you are at the mercy of thermal conductivity coefficients possibly interfering with readings.
Frankly, I wouldn't even try; 1700 mV is so far out of line I strongly suspect the device is damaged, perhaps from being overpressured and having its pressure diaphragm popped. The fact that you only got an 8 mV shift when drawing a vacuum on it furthers that suspicion.

And in any case, I wouldn't use that method to null out an offset that large; most of the time I've used it to null offsets in the range of +/- 10 mV or so.

#### vane

Joined Feb 28, 2007
189
That is disappointing you think it is damaged, I knew I was taking a bit of a risk getting a second hand sensor. I assume a sensor of this level would probably be several hundreds new so thought I'd try it.

#### swr999

Joined Mar 30, 2011
31
If you can consider a different sensor, here's a <\$15 part you might try. I've used these in several projects with good results. 100 kPa (~15 PSI) absolute, 5V single-supply. Easy to apply (3 pins +5, Vout, Gnd). Full scale output Vout 0.3-4.7V. Additional amplification not required.