REFT & REFB & Input Span problems with AD9235

Im using AD9235-20 in single-ended configuration with 1V applied to Vin-.The power supply of AVDD and DRVDD were both 3.3V.And the clk was drived by a 10.000Mhz-crystal-controlled oscillator.

The testing source came from a regulated power supply.The problem was that,when input varied from 0v to 2v,AD9235 could only accept signals ranged from 0V-1.9V(When   Vin+ was applied with a level above 1.9V,the OTR changed to ’1‘).

So I used a multimeter to check several pins.Sense&Mode pins were connected to ground.To my surprise,the voltages at REFT & REFB pins were 2.29V and 1.28V respectively.They should be 2.15V and 1.15V.

Was it wrong to using a multimeter to check the REFT & REFB pins?(Maybe the output impedance at REFT & REFB pins cannot be ignored compared to a multimeter.)

Or the power supply were too high?(I tried a 3V power supply.And the voltage at REFT & REFB pins changed to 2.1V and 1.1V respectively.)

Or the input impedance didn't match because i only applied the 1V of CM to Vin-?( I have noticed the Fig37 in datasheet of AD9235.But if i had used  AD9235 in that way,I couldn't have got the DC component in my signal.)

Can anyone help me,plz?

If needed,i can upload the picture of my schematic diagram of AD9235.

  • 0
    •  Analog Employees 
    on Jan 22, 2012 7:33 AM over 9 years ago

    Hello,

    The REFT and REFB voltages that you measure with a multimeter are well within specification.  Note, the analog  input span is determined by the differential voltage (i.e. 2x (REFT-REFB) ) across these differential reference amplifiers which are biased about mid-supply (i.e. AVDD/2 with 10% tolerance).  Since you are measuring 1 V, the input span is 2 V.  Also, if you vary the AVDD supply from 3.3 to 3.0 V (i.e. 300 mV)...........one would expect  the REFT and REFB voltages to drop by 150 mV.

    Figure 13 in the AD922x datasheet link below shows a better single-ended drive circuit (with exception that VREF should be shorted to SENSE for 0-2 V input range.  The same VREF that establishes the input voltage span of 2V is used to establish the mid-point (vs external resistors with 5% tolerance referenced to AVDD).  Try this configuration and see if you achieve better DC gain and offset error than what you measure below. Regards.

    http://www.analog.com/static/imported-files/data_sheets/AD9221_9223_9220.pdf

  • 0
    •  Analog Employees 
    on Aug 2, 2018 4:29 PM over 2 years ago
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin