differential amplifier common mode output

I moved this discussion to the Differential Amplifiers community.  Please continue the discussion here.

Thanks,

AndyR

EngineerZone Community Manager

Saravadhamu,

If what you are trying to do is to take the output of your differential output sensor and turn in into a single-ended signal, you may want to consider using an instrumentation amplifier instead. I recommend that you take a look at the AD8221.

http://www.analog.com/en/amplifiers-and-comparators/instrumentation-amplifiers/ad8221/products/product.html

It also has the ability of level-shifting the common mode of the signal. The applications information section in the datasheet contains more info on how to do this without affecting performance.

Hello,

Using a differential amplifier such as the ADA4927-1 for differential to single ended conversion is not recommend, as there is significant degradation in second harmonic distortion performance in this configuration. If this is what you are trying to do, you could look at the AD8221 instrumentation amplifier, but since you selected the ADA4927-1 to start with, you may have higher frequency requirements than what the AD8221 can deliver. Therefore I'd suggest you take a look at the AD8129/30, it is designed for differential to single ended conversion at wide bandwidths. For more information on differential amplifiers you may also want to look at our application note AN-1026.

Regards

John

Thanks for your reply.I really appreciate your answer.i want to have the slew rate to be more for my high frequency application.if the slew rate to be low,it will decrade the performance of the system.So please recommend some other Analog Devices part number having more Slew rate.

Thanks,

Saravanan Dhamotharan.

Thanks for your reply.I really appreciate your answer.i want to have  the slew rate to be more for my high frequency application.if the slew  rate to be low,it will decrade the performance of the system.So please  recommend some other Analog Devices part number having more Slew rate.

Thanks,

Saravanan Dhamotharan.

Hello Saravanan,

Do you need higher slew rate than what the AD8129/30 can provide? If so I would suggest that you look at the ADA4857-1/2 op amp and use it in a difference amplifier configuration, to go from differential input to single ended output. It has very wide bandwidth and very high slew rate.

I hope this helps.

Regards

John

HI Gustavo,

I am using differential amplifier ADA4927 to drive ADC. Differential input given to the amplifer is having 10mV differential dc offset, i.e. differential input is riding on 5mV differential dc. How can I cancel this DC offset in the output? Output of the amplifier has to be DC coupled to ADC. Suggest appropriate circuit.

Thanks and Regards,

Dwijen

Hello Dwijen,

What is the circuit configuration? What is the gain? What are the values of the feedback resistors? What is the maximum allowable offset in your application? Can the system be calibrated? Are you comfortable sharing a copy of your schematic? I think that would make it easier for us to provide a good answer.

In general, the total offset observed in the output will be a result of many factors. Commonly, input offset voltage, input bias current, source resistance and gain will directly affect it.

Best regards,

Gustavo

Hi Gustavo,

Thank you very much for your reply

We are currently in the designing phase of the circuit, so schematic is not yet ready but let me explain you how it is.

We are required to feed output of Analog Devices demodulator ADL5387 to ADC. We are feeding this outpout from ADL5387 to ADC through bufered differential amplifier ADA4927. Now demodulator (ADL5387) has got default DC offset of 5mV. As per our input, output of the device would vary from 3mV DC to 0.8V DC or 10mVp-p ac to 1.8Vp-p ac and 5mV DC would be added aditionally to these values. When AC is comming at output we can filter out DC offset through DSP filtering after digital conversion. Our problem is when output from device is DC i.e 3mV+5mV(DC offset). This would be difficult to detect as device already have 5mV DC offset. So our aim is to reduce this 5mV DC offset

Now as AC at the output of the demodulator goes upto 1.8Vp-p. So we can not provide any gain in differential amplifier cause it would bring input to the ADC out of its range 2v p-p. Differential amplifier would be unity gain and would only act as buffer in driving ADC. ADC part number is ADS62P42. Datasheet of ADC and demodulator is attached for your referrence.

I hope this explanation will make design line up and constraints more clear.

Note: ADC is present in client system and we can not replace it 

Thanks and Regards,

Dwijen

RF Design Engineer,

HCL Technologies

You can compensate for the offset (and retain the dc component of your signal) with the circuit shown at the end of this email.

Need the same RCM in both sides to retain balance. One RCM is usually grounded the other is connected to a reference voltage that will cancel input dc offset as well as the input offset of the diff-amp.

The value of RCM is large compared to Rg.

For Fixed compensation VREF can be VCC or VEE. In this case, you determine the input side that is at higher dc level and connect that RCM to ground. Apply a VREF to cancel out all nominal offsets.

For dynamic compensation, VREF can come from a DAC. In this case the value of VREF is controlled dynamically. The dc offset is measured in the digital domain at some appropriate instant ( for example at power up) when the inputs are at a known “zero” state and a correction code is input to a DAC whose output is VREF.

The accuracy of this correction can be made quite high if the value of RCM is high.

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