# Basic query regarding input range of ADC

Hi

I am selecting between many ADCs these days. Sometimes the input range of ADC is in absolute value like  ± 4V  but sometimes it is written as 2.5 Vpp. I basically need a bipolar ADC for my application but I am confused whether a 2.5 Vpp would accept negative voltage or not?. As far as I am concerned the 2.5 Vpp  could be ranging from a 0 V to 2.5 V centered around 1.25 V. or it could be from -1.25 V to + 1.25 V centered around 0V. I am stuck at this point and any kind of help would be highly appreciated.

Regards

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• Hi Abdul,

The High Speed ADCs I'm familiar with have differential inputs. Let's call these inputs VIN+ and VIN-. The amplitude of the voltage converted by the ADC is the voltage on VIN+ minus the voltage on VIN-. When the voltage on VIN- is higher than the voltage on VIN+, the ADC will "see" a negative differential voltage even though both the voltage on VIN- and VIN+ are positive.

In the case of an ADC that has in input swing of 2.5Vpp, it could very well convert a differential signal from -1.25V to +1.25V, but each voltage could still be positive with respect to ground. Here is a picture that illustrates this for a 2Vpp (±1V) differential signal:

This shows a differential signal swinging from +1Vdiff to -1Vdiff (2Vpp). The common mode voltage VCM in this case is 0.9V. All the waveforms in this picture are positive with respect to ground. This picture is representative of a full scale signal for ADCs such as the AD9253, AD9257, and others in this family.

So, if the ADC you are considering has differential inputs, it could be "bipolar" and still not convert a signal that is negative with respect to ground. I do not know of an ADC that can convert a voltage that is negative with respect to ADC ground. I believe there are ADC driver amplifiers that have negative supplies that can be driven by a negative signal.

Thank you.

Doug

• Hi Doug,

Thanks for getting back to me. I am in dire need of an ADC Evaluation Board that could convert voltage that is negative w.r.t to ground. It should have a resolution of 16 bit and sample rate ranging from 1M to 5M preferably with parallel interface. I had initially selected DC919A-F for this purpose but I am not so sure about it right now.

Regards

• Hi,

I'm not aware of a stand-alone ADC that can convert a single-ended negative-with-respect-to-ground signal. I think you'll need to design an amplifier front end where the amplifier has a negative supply and can receive the negative signal.

What is the negative voltage value you need to convert? Also, is it DC or close to DC?

Thanks,

Doug

• Hi

I need to convert  ± 1 V for my application. I am basically trying to design a receiver where the voltage from the RF front end is minute and swings around a common mode voltage of 0 V.  Also could you care to elaborate on amplifier front end point I couldn't quite comprehend it. Thanks for your help I really appreciate it.

Best Regards

• Hi Abdul,

What is the maximum possible frequency of your signal?

The amplifier front end I'm thinking of is using an amplifier to convert your single ended signal to differential, so it could be interfaced to an ADC. One example is the AD8476. Please see the SINGLE-ENDED-TO-DIFFERENTIAL CONVERSION section of the AD8476 datasheet. Figure 54 shows a conceptual diagram of the setup to receive a single-ended signal.

Thanks,

Doug

• Hi Doug,

I am sampling an AM modulated signal of 455 KHZ and I want an ADC for that. The information regarding the differential amplifier was helpful. Thanks

Regards

• Hi Abdul,

455kHz is no problem. You do not need a high speed ADC for that frequency.

You can use ADI DiffAmpCalc to play with different initial amplifier configurations. The AD8476 is modeled in DiffAmpCalc.

Thanks,

Doug