AMC7823 Datasheet, PDF (31/48 Pages) Texas Instruments – ANALOG MONITORING AND CONTROL CIRCUIT

AMC7823
www.ti.com
SLAS453A – APRIL 2005 – REVISED OCTOBER 2005
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
10-kΩ 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 [ (10 kΩ) ×
(C_FILTER) ]. The resistor also allows an external reference applied to pin 21 to override the internal reference,
provided it can drive the 10-kΩ load.
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 10-kΩ 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 +1.25 V. When GREF is set to '1', the internal reference voltage is +2.5 V.
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 10 kΩ. For
example, if SREF and GREF are both cleared to '0' and a 2.5-V external reference is applied, the AMC7823
must unnecessarily sink 125 µA.
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