Datasheet

AD7927 Data Sheet
Rev. D | Page 16 of 28
For ac applications, removing high frequency components from
the analog input signal is recommended by use of an RC low-
pass filter on the relevant analog input pin. In applications where
harmonic distortion and signal-to-noise ratio are critical, the
analog input should be driven from a low impedance source. Large
source impedances significantly affect the ac performance of the
ADC. This may necessitate the use of an input buffer amplifier.
The choice of the op amp is a function of the particular application.
When no amplifier is used to drive the analog input, limit
the source impedance to low values. The maximum source
impedance depends on the amount of THD that can be
tolerated. The THD increases as the source impedance
increases, and performance degrades (see Figure 8).
R1
D1
D2
A
V
DD
V
IN
C2
30pF
C1
4pF
CONVERSION PHASE: SWITCH OPEN
TRACK PHASE: SWITCH CLOSED
03088-016
Figure 16. Equivalent Analog Input Circuit
ADC TRANSFER FUNCTION
The output coding of the AD7927 is either straight binary
or twos complement, depending on the status of the LSB in
the control register. The designed code transitions occur at
successive LSB values (that is, 1 LSB, 2 LSBs, and so forth).
The LSB size is REF
IN
/4096 for the AD7927. The ideal transfer
characteristic for the AD7927 when straight binary coding is
selected is shown in Figure 17, and the ideal transfer characteristic
for the AD7927 when twos complement coding is selected is
shown in Figure 18.
000…000
0V
ANALOG INPUT
111…111
000…001
000…010
111…110
•
•
111…000
•
011…111
•
•
1LSB
1LSB = V
REF
/4096
+V
REF
– 1LSB
NOTES
V
REF
IS EITHER REF
IN
OR 2 × REF
IN
.
03088-017
Figure 17. Straight Binary Transfer Characteristic
ADC CODE
ANALOG INPUT
100…000
011…111
100…001
100…010
011…110
•
•
000…001
111…111
•
•
000…000
1LSB = 2 × V
REF
/4096
+V
REF
– 1LSB–V
REF
+ 1LSB
V
REF
– 1LSB
03088-018
Figure 18. Twos Complement Transfer Characteristic with REF
IN
± REF
IN
Input Range
HANDLING BIPOLAR INPUT SIGNALS
Figure 19 shows how useful the combination of the 2 × REF
IN
input range and the twos complement output coding scheme
is for handling bipolar input signals. If the bipolar input signal
is biased about REF
IN
and twos complement output coding is
selected, then REF
IN
becomes the zero code point, −REF
IN
is
negative full scale and +REF
IN
becomes positive full scale, with
a dynamic range of 2 × REF
IN
.
R3
R2
R4
AD7927
V
DOUT
011…111
000…000
100…000
(= 0V)
0
V
V
V
REF
0.1µF
R1
R1 = R2 = R3 = R4
AV
DD
REF
IN
V
IN
0
V
IN
7
V
DRIVE
TWOS
COMPLEMENT
V
DD
V
DD
DSP/
MICROPROCESSOR
+REF
IN
REF
IN
–REF
IN
(= 2 × REF
IN
)
03088-019
Figure 19. Handling Bipolar Signals