reference of AD9245

We use AD9245 to sample the signal detect single end amplifier with output voltage from 0 to 2 volts.  We connect this output to Vin+. My question is that should we connect the Vin- to ground or to a voltage reference of 1 volt. If we connect the Vin- to ground, the differential input of AD9245 will be from 0 to 2 volts. If we connect the Vin- to 1 volt reference,  the differential input of AD9245 will be from -1 to 1 volt.  Could you please tell us which solution is right or both are right?

  • 0
    •  Analog Employees 
    on Dec 12, 2017 6:08 PM

    Hello,

    For a 2 V input single-ended input span ranging from 0 to +2 V, the VREF output must be configured for 1 V while the VIN- pin is connected to VREF via the same value resistor (i.e. 33 ohms) that is used to connect VIN+ to the amplifier source which is producing the 0 to 2 V input signal.

  • thanks

    we follow this setting, and 1V give VIN- to have 0-2V input as ±1V digital output for servo control.

    target is to match VIN+=VIN- as 1V. 

    by now we get +60mV different between VIN+ with VIN-  when servo is working(VIN+ always bigger than VIN-).

    is that means the VIN+ reading is 6% less than real voltage?

    if I change target half, I will have +30mV VIN+ bigger than VIN-.

    because the code only check AD9245 is 0 or not, reduce output when bigger than 0 and increase output when less than 0. 

    both VIN+ and VIN- has 33 ohms input resistor.

    VREF connect to VIN-, there is 1K to ground before 33 ohm.

    input through 33 ohm to VIN+, no resistor between it with ground.

    could you please help check if any connection worn?

    thanks again.

  • 0
    •  Analog Employees 
    on Apr 30, 2018 10:25 PM

    Hello,

    If you run AD9245 at 1V internal reference by connecting SENSE pint to ground and bias VIN- with 1V VREF, you shouldn't need 1K ohm to ground on VIN- side.  The source impedance on each inputs should be well matched in order to achieve the best possible performance.

    Questions are : When you say " change target half", what does that mean? Does it mean to set VIN+=0.5V = 1/2 VIN- ?

                              How do you set the output common mode voltage on AMP output(=VIN+ of AD9245)?

                             

    The digital code "0" covers within +/- 1/2 LSB (61uV here) and offset error of AD9245 on datasheet is typically +/-0.3% and Max +/-1.2%  FSR @ 1.0V internal reference. Offset error at Max 1.2% case is 2V_FS*1.2*0.01=24mV. So it doesn't explain 60mV fully here. Please let me know if there is any change without 1K ohm on VIN- side.

    Thanks

    Tony