Very basic instrumentation amp help

Thread Starter

cps13

Joined Feb 25, 2013
9
Hi,

I am after some help designing an instrumentation amp - this is not a college project etc so I am not cheating!

I am trying to simulate the working of a TI INA333 instrumentation amp IC using multisim 13.0 with little success. Please can someone point out what I am sure is my obvious mistake?

My understanding is that the IC should output 0-5V with a varying input.

I tried to eliminate possible problems using a multimeter on a simple 12V battery but get no reading when simulation either.

Many thanks
 

Attachments

Thread Starter

cps13

Joined Feb 25, 2013
9
OK - so I have now grounded the battery and get an output of -599 micro volts when the dc input is set to 1V and R1 is 500 ohm.

Any ideas?

Thanks
 

SgtWookie

Joined Jul 17, 2007
22,230
You need to post your updated schematic, or we can't be certain of the modifications you've made. This will save time and give you better answers.
 

Thread Starter

cps13

Joined Feb 25, 2013
9
I have attached a new screenshot. I have made a change in that it now has a wheatstone bridge attached to provide the input.

thanks
 

Attachments

AnalogKid

Joined Aug 1, 2013
10,943
All of the answers are in the datasheet.
What is pin 4?
What is the purpose of R5?
What are the two input voltages?
What is the input voltage range of the device?

ak
 

Thread Starter

cps13

Joined Feb 25, 2013
9
All of the answers are in the datasheet.
What is pin 4?
What is the purpose of R5?
What are the two input voltages?
What is the input voltage range of the device?

ak
Pin for is V- - mine is connected to ground
R5 is your gain resistor - I don't understand why you have highlighted this
input voltages are +1.8V to +5.5V - my input is 5V
input range is are -100mV to +100mV - I have powered this as a normal wheatstone bridge is powered to provide a mV output.

I realised I hadn't changed the value of R3 to 350 ohm. It was set to 1k. I have corrected this.
 

AnalogKid

Joined Aug 1, 2013
10,943
You must have a different datasheet from the one on TI's website. Here is what I see.

Pin 4 is the V- supply, but you have it tied to a 1K resistor to ground.

R5 is not the gain resistor; the pot marked GAIN is the gain resistor.
I highlighted R5 because it is in the negative power leg of the amp, where it should not be.

If your intent was to adjust the differential gain of the amp by adding a resistance (R5?) to the reference input (pin 5), note that that will seriously compromise the diff in diffamp. If you are trying to add an offset to the output, note that the impedance at the reference input must be very low to maintain common mode rejection.

Changing R3 to 350 ohms balances the bridge, but it better be *very* well balanced. With the 500 ohm gain pot at 50% rotation (250 ohms), the circuit gain is 401, so your max differential input voltage before positive clipping is 12.3 mV.

ak
 
Top