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Voltage Reference technical note
AD9628
INTERNAL ADC VREF SOLUTION
To GND =VREF int. generator
To +1.8V = external VREF
MASTER ADC
VREF generator
SLAVE ADC
VREF LOAD
VREF
SENSE
+1.8V
VREF
ADC
SENSE
+1.8V
VREF
ADC
SENSE
+1.8V
VREF
ADC
SENSE
+1.8V
VREF
ADC
SENSE
GND
ADC
Internal Voltage Reference considerations
Typical drift characteristics of the internal reference mode
Error is from -20 to +60degrees about 5 mV on 1 Volt
LOAD CURRENT (5 ADC)
7.5 Kohm x 5 ADC
LOAD IS 1.5 Kohm
The current from VREF
master ADC is 0.6 mA
About -0.2% or -2mV on 1V
External Reference generator
Example of solution
Precision zener diode
V out 1V
Precision resistors
SLAVE ADC
VREF LOAD
VREF
SENSE
+1.8V
VREF
ADC
SENSE
+1.8V
VREF
ADC
SENSE
+1.8V
VREF
ADC
SENSE
+1.8V
VREF
ADC
SENSE
+1.8V
ADC
Conclusions
Voltage reference is very important for the precision of analog conversion
Internal ADC generator for the common reference is a solution with
very low thermal drift and the error for the load current (five ADC) is
about 0.5 % (5mV on 1V)
The solution with external generator must have better features than
internal generator and the circuit should be appropriate and stable.
The proposed external circuit solution requires an appropriate thermal exscursion test.
However, a solution with external generator could be provided and not mounted on the board.