AD7793_current output compliance voltage and impedance

Document created by analog-archivist Employee on Feb 23, 2016
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Here is another issue from the customer. They are using AD7793 , but found the
output of current source will change according to the load resistance. But this
only appears on the Gain setting =1 and 2 and the current source works well 
when the Gain>4.

           Below is the test result from the customer. Have ever heard about
this issue? Could you suggest why the output current is decreasing according to
the load. Thanks a lot.
    
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they are using RTD test circuit
Output of current source 210uA?sampling rate = 62.5Hz?bipolar mode?Channel 1 ?
continuous sampling ?single read
When the G=1
Load resistor        Output Reference Voltage (Iout X R)
0                                   2.32V
?=2.068K             2.32V
7.95K                 2.01V
?=11.43K            1.83V
When the load is smaller than 2.068K, the Reference voltage is stable. After
that, when the load is increasing the Reference voltage decrease. when the load
is bigger than 11.43K, the output is stable again no matter what the load
resistor value are.  When the Gain is 2, the issue still exist. But when G is
bigger than 4, the problem does not show.

 

Is the load resistor connected between AIN+ and AIN-. What power supply voltage
is being used. The 210 uA excitation currents have an output compliance of 
AVDD - 0.65 V. So, the voltage at IOUT must be less than AVDD - 0.65V. If AVDD
= 5V, then the voltage at IOUT must be less than or equal to 4.35V.

The reference voltage equals 2.32V. So, at a gain of 1, the maximum resistance
between AIN+ and AIN- is (4.35V-2.32V)/210uA = 9.6K. The excitation current
value of 210uA is a typical value. The customer should allow some headroom as
the excitation current will vary from part to part.

As the gain increases, this resistance will be lower. So, at a gain of 4, for
example, the maximum resistance between AIN+ and AIN- is 2.4K.

Can you check with the customer to ensure that the output compliance spec is
being met.

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