Is 2.5V ok as the input common-mode voltage for AD7760's VinA+/VinA- input?

Hi, 

We're using AD7760 in our design and wonder about the input common-mode voltage for VinA+/VinA-.

In the example(figure 52) of the datasheet, the input to VinA+/VinA- is 0±2.5V with common-mode voltage 0V.

We want to use 2.5±2V differential signal as input to VinA+/VinA- with common-mode voltage 2.5V.

Since there's no range specification of this common-mode voltage in the datasheet, we want to check if 2.5V can be used as input common-mode voltage to VinA+/VinA-.

Thanks a lot.

ZCH  

  • 0
    •  Analog Employees 
    on Jan 18, 2021 1:44 PM 1 month ago

    Hi, 

    The datasheet mentioned on the Modulator input section (p.19) , the input must sit on a common mode of Vref/2. So I guess if you have 4.096Vref  for example then the common mode input must be 2.048V, then the maximum input to each differential modulator input is up to 80% of VREF. 

    As for the case of VINA+, VINA- these are inputs to differential amplifier which always biases the output to the optimum common mode of Vref/2. The signal is also scaled to give the maximum allowable voltage swing with this reference value. 

    Thanks,

    Jellenie

  • Jellenie,

    Thanks for you information.

    I know the input to the Modulator (VIN+/VIN-) should have a common mode of Vref/2 and the differential amplifier will biases the output to the common mode of Vref/2.

    What I'm asking is the input common mode of VINA+/VINA-. Can I use 2.5V common mode voltage as input to the differential amplifier?

    The input common mode voltage is zero in Figure52 of datasheet, but we want to use 2.5V as the input common voltage.

    Thanks.

  • 0
    •  Analog Employees 
    on Jan 18, 2021 3:17 PM 1 month ago in reply to ZCH

    Hi, 

    The output of the amplifier will be the input to the ADC/modulator. So ideally, the current specified amplifier configuration is meant to maximize the allowable modulator input range and at the same time meet the required common mode input of the modulator with +/-2.5V VINA+/VINA- input biased around ground. 

    So having a 2.5V common mode input on the specified configuration (using a normal mode component values) will not meet the allowable modulator input range. So the answer to your questions is no. 

    However, if you are trying to modify the amplifier circuitry (different component values) and be able to meet the ADC/modulator input range and at the same time the required common mode input of Vref/2 with your VINA+/VINA- input then the answer to your question could be yes. But in this mode, we cannot guarantee the performance as it was not specified and tested. 

    May I know what's the need to bias the input at 2.5V? May I know your application? 

    Thanks,

    Jellenie

  • Hi,

    Actually we use ADA4945-1 as the driver amplifier before AD7760. If the input to VINA+/VINA- is biases around ground, negative supply is needed for ADA4945-1. Actually we want to use +5/0 as supply for +Vs/-Vs to ADA4945-1 similar as following application found at ADI website. (I think the internal structure of AD7768-1 is similar to AD7760)

    As to what you mentioned of "to maximize the allowable modulator input range and at the same time meet the required common mode input of the modulator"

    1. the modulator input range is 3.275Vp-p. with 2.5V±1.5V input to VINA+/VINA- at gain of about 1.1 can maximize the allowable modulator input range.

    2. the required common mode input of the modulator is meet by the internal amplifier itself (The differential amplifier always biases the output signal to sit on the optimum common mode of VREF/2, in this case 2.048 V.(datasheet p26)). Will the common mode voltage at VINA+/VINA- affects the output common mode voltage?

    So I don't understand your conclusion of a nonzero common mode voltage(e.g. 2.5V) can't meet the requirement of the modulator.

    BTW, in the datasheet's "Absolute Maximum ratings", the range of VINA+, VINA− to GND1 is −0.3 V to +6 V. In P26 Figure52 the input to VINA+/VINA- is as low as -2.5V. It seems contradictory.

    Thanks.

    ZCH

  • 0
    •  Analog Employees 
    on Jan 19, 2021 9:15 AM 1 month ago in reply to ZCH

    Hi, 

    Thanks for this details. 

    First I would to take note that AD7768-1 core is quite different from AD7760 thus they also have different specifications. AD7760 is an older version though I agree that they are quite similar as they are both wideband Sigma delta ADCs. So I understand that you have an extra analog front end before the internal amplifier, however the purpose of the internal amplifier is to reduce the external signal conditioning requirements, unless your input is single ended and the ADC is only specified with fully differential input signal, so an extra amplifier in front is necessary to convert your single ended input to differential. 

    The reason why I am asking the need for a 2.5V common mode input is because AD7760 is specified for a VREF/2 common mode input. So if you have 4Vref the common mode input must be 2V. So if you intend to use 2.5V common mode and directly connected it to the modulator then it violates the optimum common mode input. And I cannot guarantee how it will affects the conversion results as the modulator output code is also referred with a common mode of VREF/2. 

    If you intend to use the internal amplifier configured as shown in Figure 50 and using the external components as listed in Table 8. So yes the input source common mode will be added to the internal amplifiers common mode. Providing 2.5V will clipped the inputs as this circuitry already biased the signal at 2.048V.

    So if you have an evaluation board, I'll suggest to try and explore how the part works. You can try to input +/-2.5V at J1 i think that will go to VA+/VA- input and then try to probe VA+ to GND you'll notice that the common mode there is above ground same as VA- to GND. This is due to the amplifier configuration. The input can be at the minimum of -2.5V at the terminals VA+ and VA-.This is because the output of the differential amplifier VoutA+ and VoutA- are always biased to 2.048V. –this means that this DC bias will feed back through the components RFB to the in input pins of the amplifier. 

    Thanks,

    Jellenie