Flux sensor interface

Hi ChrisvdA ,

Good day. I would recommend AD8422 for your application. AD8422 is a "High Performance, Low Power, Rail-to-Rail Precision Instrumentation Amplifier". Unfortunately, I would suggest you implement your setup in a dual supply since the expected output (-10 V to 10 V) would violate the output range as well as the IVR if you implement it in a single supply. You can use the InAmp Diamond Plot tool to further check your configuration. Here's the link for it: tools.analog.com/.../

You can use an FDA to make your output differential. Thank you.

Regards,
Gilbeys

Hi Gilbeys,

I am aware that I need dual supply out of incoming +24V  but I already found a device for that. The AD8422 looks interesting but I also found the LTC6373 at your company which even has a little bit better specs wrt CMMR. Price is not much different from the AD8422 B grade (A grade is cheaper and maybe good enough for our application?). Besides the LTC6373 has already a differential output so maybe I can use this to connect this to an external analog input module (via a cable). Or would you prefer to use extra unity gain buffers (dual opamp) when you connect the LTC6673 to an external input module (with differential analog input range -10V..+10V)? Or do you think the AD8422 would be better for magnetic flux measurements?

Best regards Chris

Hi Gilbeys,

In the meanwhile I did some investigations and simulations in LTSpice with the AD8422. However some questions about this. Maybe you can help us with some advice. Our output has to be -20V..+20V differential because we want to connect this with an external analog module (part of our control system) that has a differential input range of -20V..+20V (so one pin at +10V and thje other at -10V or vice versa). So I make a dual +12V/-12V supply out of our incoming +24V with +15V/-15V DC/DC and low noise LDOs for +12V/-12V (we only need a few mA)

See LTspice simulation below for white noise simulation where I could manage to get this output range and made an offset compensation (we want to use multi turn potmeter for adjustment). For this simulation I only used 1 opamp at the output but in fact we also need a second one for inverting output (or using a single ended to diff amp). The problem here was that the inverting opamp has influence on the output amplitude because of the resistor feedback (so I had no high impedance input like the non inverting part) and therefore I removed the inverting output opamp 

I also did some noise simulation by adding white noise of 1 MHz at the voltage supplies (100uVtt which is more then the LDO spec )we are using) and 10uAtt noise reference current.

This results in a noise around 10mVtt at the output of the dual opamp mainly cause by  the amplifier when I add a RC filter (-3dB around 15 kHz) at the output of the AD8422. 

I also tried with a RC filter at the input between sensor and amplifier but I understood that the resistor can not be too large and has not much effect in the LTspice simlation compared with a filter at the output. Besides in most applications with a strain gauge (wheatstone bridge) I see no filter between sensor and amplier. Adding series  resistors also introduces extra noise because or the resistor noise that will be amplified.

Some questions about this:

• Do you think this will work as supposed in the simulation circuit below?

• Would you prefer AD8422 or LT6372-1? The noise and CMMR figures are not much different. In the datasheet I see EMI filtering at the inputs of the LT6372-1 so I don’t know if this could have some advantages for flux/ Hall sensor measurements (it is a resistive wheatstone bridge)?

• For the reference we want to use an accurate 3V voltage reference with opamp feedback to the ground pin because the sensor supplier advices us to use a current source and not  a constant voltage at the sensor. I don’t know if you have a reference design for such a current source with low noise and +12V input supply?

• Do you think we can adjust the offset with a positive and negative +12V/-12V (because I have to set the ref voltage a little bit below 0V) or do you think it would be better to use the +3V ref at one side of the resistor divider and making a -3Vref with an inverting opamp for the other side of the resistor divider so you get a more accurate reference voltage at the AD8422 REF pin?

• At the output I used a standard opamp tin the simulation. I also fiound the ADA4922 single ended to differential amp that can operate at +12V/-12V supply (there are not many diff amps that can handle this voltage). Do you expect better results with that compared with 2 opamps (or dual opamp) with low noise and low offset?

• What do you suggest about filtering?We expect that 20kHz would be sufficient for this but we can always change the filter frequency. Do you think filtering is better at the output of the AD8422 or between the bridge and the input of the AD8422 (in most applications I do not see RC filters at the input of the amplilfier)

Best Regards

Chris