AMC7823_15 Datasheet, PDF (32/55 Pages) Texas Instruments – ANALOG MONITORING AND CONTROL CIRCUIT

AMC7823
SLAS453F – APRIL 2005 – REVISED MARCH 2012
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REFERENCE
The AMC7823 requires a reference voltage to drive the ADC, the DACs, and the precision current source. It can
accommodate the application of an external reference voltage, or it can supply reference from an internal
bandgap voltage circuit as shown in Figure 48. Pin 21, EXT_REF_IN, is common to either source. An internal
10kΩ resistor connects the internal reference source to pin 21. This pin provides a point for noise filtering by
placing a capacitor, if desired, from the pin to analog ground. The time constant of this filter is [ (10kΩ) ×
(C_FILTER) ]. The resistor also allows an external reference applied to pin 21 to override the internal reference,
provided it can drive the 10kΩ load. The filter capacitor can be 10μF when the internal reference is selected.
When using an external reference, it is recommended to use a buffered reference (such as the REF5025).
Bandgap
Reference
1.25 V
2.5 V
Bit
GREF = 0
10 kW
Bit
SREF = 1
C Filter
21
A1
(Refer to diagram
of Current Source,
Buffer and
Figure 49)
Controller
A2
To DACs
To Precision
Current Source
Figure 48. Reference
To ADC
Logic bits SREF and GREF of the AMC Status/Configuration Register specify operation of the internal reference
and also should be considered when applying an external reference, as explained below. These bits also provide
information to the precision current source and are used in configuring that source (refer to the Precision Current
Source section for details).
When SREF is cleared to '0' (the default or power-on reset condition), the internal reference is selected and
connected to pin 21 by the 10kΩ resistor. When SREF is set to '1', the internal reference is de-selected and
disconnected from pin 21. Pin 21 then floats unless an external reference is applied.
The bit GREF selects one of two pre-set values for the internal reference voltage. When GREF is cleared to '0',
the internal reference voltage is +1.25V. When GREF is set to '1', the internal reference voltage is +2.5V.
It is recommended to always set SREF to '1' when using an external source; otherwise, the external reference
must sink or source a current of value equal to the difference in the reference voltages divided by 10kΩ. For
example, if SREF and GREF are both cleared to '0' and a 2.5V external reference is applied, the AMC7823 must
unnecessarily sink 125μA.
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