WM8200
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PROGRAMMABLE-GAIN AMPLIFIER
VP is amplified by the PGA and fed into the ADC as a differential voltage VQ = VQ+ - VQ-
VQ = Gain ×VP = Gain × VINP − VINN
( )
The PGA gain defaults to 1.0 at power-up, but can be programmed from 0.5 to 4.0 in steps of 0.5.
ANALOGUE-TO-DIGITAL CONVERTER
Regardless of the reference configuration, VQ is digitised against ADC Reference voltages REFT and
REFB, full scale values of VQ being given by:
REFT − REFB
REFT − REFB
VQFS
=
VQZS = −
and zero scale by
2
2
Attempts to convert VQ voltages outside the range of VQZS to VQFS are signalled to the application by
driving the OVR output pin high when the conversion result is output. If VQ is less than VQZS, the ADC
output code is 0. If VQ is greater than VQFS, the output code is 1023.
SIGNAL CHAIN SUMMARY
Combining the above equations to find the input voltages [AINP – AINN] that correspond to the limits
of the ADCs valid input range gives:
(
REFB − REFT
)
(REFT − REFB)
≤ [ AINP − AINN ] ≤
(
2×Gain
)
2×Gain
( )
Therefore the input signal span is given by:
REFT − REFB
AINP − AINN =
Gain
In order to match the ADC input range to the input signal amplitude, REFT and REFB should be set
such that:
REFT − REFB = ( AINP − AINN ) ×Gain
ADC REFERENCE MODES
The WM8200 references REFT and REFB can be driven from external (off-chip) sources or from the
internal reference generation/buffer circuit. The mode of operation is selected by the voltage applied
to the MODE pin. These are summarised and explained in Table 2.
Note that the internally generated ADC references are intended solely for WM8200 internal use and
REFT and REFB must not be used as voltage references for any other device in the application.
MODE
PIN
MODE
FUNCTION
COMMENTS
AGND
Full external
On-chip reference generator and reference buffer are
not used.
REFT = external
REFB = external
AVDD
Top/Bottom
On-chip reference generator is not used. Reference
buffer centers external reference voltages around
AVDD/2.
AVDD +
(
REFTS − REFBS
)
)
REFTF =
REFBF =
2
AVDD −
(REFTS − REFBS
2
Table 2 WM8200 Reference Generation Modes
PP Rev 1.22 March 2002
8
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
