Differential unity gain amplifier

[shshprsd]

I am trying to drive 50 Ohm load from output of a accelerometer.
The output of accelerometer is a differential signal max 3V across common mode 2.5V.
I used a differential to single ended unity gain amplifier as shown in fig, reference set to 2.5 V.
but with this circuit I am getting output swing of +-1 V across common mode. For differential input voltage is > 1V output is stuck at 2.5 +1 V or 2.5 - 1V dependent upon polarity of IN+ - IN-.

I seperatly checked the differential to single ended unity gain amplifier with function generator, I am getting the Specified Output voltage swing.
can anyone tell What is issue when driving accelerometer.

 

[erikl]

Does your amp dispose of an input voltage range from 1 to 4V, and an output voltage range of 2.5±3V , i.e. -0.5 .. 5.5V ? Because that's what you need for a max. 3V differential input to 3V single ended output amplifier at gain=1 and a center voltage of 2.5V.

Amp power supply? Output load?

 

[shshprsd]

From the accelerometer I am getting -1 to +3V differential output across common mode 2.5V.

but at the output of opamp I am getting a unity gain response for input ranging from -1V to +1V with ref to opamp ref voltage(2.5V).

Also one experment I did setting different reference voltage for opamp.

1. with ref 2.5, getting unity gain response for input 2.5-1V to 2.5 +1V

2. with ref 1.5, getting unity gain response for input 1.5-1V to 1.5 +1V

2. with ref 3.5, getting unity gain response for input 3.5-1V to 3.5 +1V.

 

[erikl]

Also one experment I did setting different reference voltage for opamp.

1. with ref 2.5, getting unity gain response for input 2.5-1V to 2.5 +1V

2. with ref 1.5, getting unity gain response for input 1.5-1V to 1.5 +1V

2. with ref 3.5, getting unity gain response for input 3.5-1V to 3.5 +1V.

Seems ok.

From the accelerometer I am getting -1 to +3V differential output across common mode 2.5V.

but at the output of opamp I am getting a unity gain response for input ranging from -1V to +1V with ref to opamp ref voltage(2.5V).

Seems there is an output saturation effect. Again: what are your opAmp supply voltages, and your load resistance?

As for +3V differential input voltage the output voltage should go to +5.5V , you should have a positive supply voltage of at least 7..8V , depending on the output range capability of your opAmp, and your load resistance.

 

[shshprsd]

on Load I have a 50 Ohm scope, the max rated supply for opamp I am using is 5V, Ok I will try 7 V supply voltage I guess 2V exess will not harm device.

 

[FvM]

I guess 2V exess will not harm device.

5V OPs have usually a maximum functional supply voltage specification of 5.5 V. Between this voltage and maximum rating (the permanent damage limit), correct operation isn't guaranteed.

Regarding output swing with 50 ohm load, did you review the output current specification in the datasheet?

Driving a cable without a series resistance (e.g. 50 ohm) is a bad idea anyway.

 

[shshprsd]

Regarding output swing with 50 ohm load, did you review the output current specification in the datasheet?

Driving a cable without a series resistance (e.g. 50 ohm) is a bad idea anyway.

Yes with opamp supply voltage 6.5 Volts I am able to getting unity gain response for the required input range. But on board have 5 V max. so availing onboard 6.5 V will be an issue.

also the rated output current for op-amp is +-80mA.
with the rated cutrrent it can drive a 50 Ohm load upto 4V, my output is within this limit so I guess that should not be an issue.

Like most of pattern generators driving 50 Ohms have 50 ohm series resistors, but they produce double the Vpp at source to have specified Vpp at 50 Ohm scopes,
so If I use a 50 ohm series resistor then I will get half output swing at 50 ohm scope, so for producing a unity gain response at output should I use gain of 2 rather than unity gain.