AD7927 Analog Devices, AD7927 Datasheet - Page 16

• Manufacturer Part Number: AD7927
• Description: 8-Channel, 200 kSPS, 12-Bit ADC with Sequencer in 20-Lead TSSOP
• Manufacturer: Analog Devices

Specifications of AD7927

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 200kSPS
• Interface: Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: Uni (Vref),Uni (Vref) x 2
• Adc Architecture: SAR
• Pkg Type: SOP

AD7927
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).
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
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.
V
IN
4pF
C1
Figure 16. Equivalent Analog Input Circuit
IN
D1
D2
0V
AV
/4096 for the AD7927. The ideal transfer
V
DD
CONVERSION PHASE: SWITCH OPEN
TRACK PHASE: SWITCH CLOSED
V
R1
0.1µF
R3
R2
30pF
C2
R1 = R2 = R3 = R4
V
REF
R4
R1
Figure 19. Handling Bipolar Signals
Rev. C | Page 16 of 28
REF
V
V
IN
IN
0
7
AD7927
IN
AV
DD
HANDLING BIPOLAR INPUT SIGNALS
Figure 19 shows how useful the combination of the 2 × REF
input range and the twos complement output coding scheme
is for handling bipolar input signals. If the bipolar input signal
is biased about REF
selected, then REF
negative full scale and +REF
a dynamic range of 2 × REF
V
DOUT
DRIVE
Figure 18. Twos Complement Transfer Characteristic with REF
COMPLEMENT
+REF
–REF
111…111
111…110
111…000
011…111
000…010
000…001
000…000
REF
TWOS
011…111
011…110
000…001
000…000
111…111
100…010
100…001
100…000
Figure 17. Straight Binary Transfer Characteristic
•
•
•
•
•
IN
IN
IN
•
•
•
•
NOTES
V
–V
(= 2 × REF
(= 0V)
REF
0V
IN
REF
IN
IS EITHER REF
becomes the zero code point, −REF
1LSB
and twos complement output coding is
MICROPROCESSOR
+ 1LSB
V
V
IN
DD
DD
)
Input Range
V
IN
DSP/
IN
REF
ANALOG INPUT
ANALOG INPUT
.
becomes positive full scale, with
1LSB = V
– 1LSB
IN
1LSB = 2 × V
OR 2 × REF
011…111
000…000
100…000
+V
+V
REF
REF
REF
– 1LSB
/4096
– 1LSB
IN
REF
.
/4096
Data Sheet
IN
± REF
IN
is
IN
IN