ADUC848: Converting a small bipolar signal.

Document created by analog-archivist Employee on Feb 23, 2016
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I have external reference 2,5V, ADR441, and I need to measure voltages in the
range of +-80mV. Is it possible to do if the signal from my source is going to
AIN1 and  AINCOM or does it need go to AIN1 and AIN2?

 

You need to convert a bipolar signal (+/-80mV). The ADuC848 cannot measure
voltages that go 80mV below GND so if your +/-80mV source is referred to Ground
you will have a problem interfacing directly to the ADuC848.

Usually, such small signals come either from a resistive "wheatstone" bridge
(as shown connected to pins 15 and 16 in figure 71 page 93 of ADUC848
datasheet, or from a sensor which can be biased at a voltage above ground
(thermocouple, RTD). In both of these cases the differential voltage can be
e.g.  +/-80mV but the common mode voltage lies between VDD and ground.

For a wheatstone bridge, you would normally use a fully differential connection
(signal connected between AIN1 and AIN2 for example)

For a floating sensor, you can either use a fully differential connection or a
pseudo-differential connection (signal connected between AIN1 and AINCOM).

In both these cases, all the input voltages need to lie between GND and AVDD at
all times. If you use the on-chip buffer, there is an additional constraint
that the absolute voltage on AIN1 and AIN2,AINCOM must remain in the range AGND
+ 100mV to AVDD - 100mV.

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