EVAL-AD7923CB2 AD [Analog Devices], EVAL-AD7923CB2 Datasheet - Page 12

• Manufacturer Part Number: EVAL-AD7923CB2
• Description: 4-Channel, 200 kSPS, 12-Bit ADC with Sequencer in 16-Lead TSSOP
• Manufacturer: AD [Analog Devices]

AD7923
When the ADC starts a conversion (see Figure 5), SW2 will
open and SW1 will move to position B, causing the comparator
to become unbalanced. The Control Logic and the capacitive
DAC are used to add and subtract fixed amounts of charge from
the sampling capacitor to bring the comparator back into a
balanced condition. When the comparator is rebalanced, the
conversion is complete. The Control Logic generates the ADC
output code. Figures 7 and 8 show the ADC transfer functions.
Analog Input
Figure 6 shows an equivalent circuit of the analog input struc-
ture of the AD7923. The two diodes D1 and D2 provide ESD
protection for the analog inputs. Care must be taken to ensure
that the analog input signal never exceeds the supply rails by
more than 200 mV. This will cause these diodes to become
forward-biased and start conducting current into the substrate.
10 mA is the maximum current these diodes can conduct with-
out causing irreversible damage to the part. Capacitor C1 in
Figure 6 is typically about 4 pF and can primarily be attributed
to pin capacitance. The resistor R1 is a lumped component
made up of the on resistance of the track-and-hold switch and
also includes the on resistance of the input multiplexer. The
total resistance is typically about 400 W. Capacitor C2 is the
ADC sampling capacitor and has a capacitance of 30 pF typi-
cally. For ac applications, removing high frequency components
from the analog input signal is recommended by using 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 will significantly affect the ac perfor-
mance of the ADC. This may necessitate the use of an input
buffer amplifier. The choice of the op amp will be a function of
the particular application.
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum source
impedance will depend on the amount of total harmonic distortion
(THD) that can be tolerated. The THD will increase as the
source impedance increases and performance will degrade.
(See TPC 5.)
V
V
IN
IN
AGND
0
3
.
.
V
IN
Figure 6. Equivalent Analog Input Circuit
4pF
C1
Figure 5. ADC Conversion Phase
SW1
A
B
D1
D2
AV
DD
SW2
4k
CONVERSION PHASE: SWITCH OPEN
TRACK PHASE: SWITCH CLOSED
COMPARATOR
R1
30pF
C2
CAPACITIVE
CONTROL
LOGIC
DAC
–12–
ADC TRANSFER FUNCTION
The output coding of the AD7923 is either straight binary or
twos complement, depending on the status of the LSB in the
Control Register. The designed code transitions occur at succes-
sive LSB values (i.e., 1 LSB, 2 LSBs, and so on). The LSB size
is REF
for the AD7923 when straight binary coding is selected is shown
in Figure 7, and the ideal transfer characteristic for the AD7923
when twos complement coding is selected is shown in Figure 8.
Figure 8. Twos Complement Transfer Characteristic with
REF
Handling Bipolar Input Signals
Figure 9 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
TYPICAL CONNECTION DIAGRAM
Figure 10 shows a typical connection diagram for the AD7923. In
this setup the AGND pin is connected to the analog ground plane
of the system. In Figure 10, REF
2.5 V supply from a reference source, the AD780, to provide an
analog input range of 0 V to 2.5 V (if RANGE bit is 1) or 0 V
to 5 V (if RANGE bit is 0). Although the AD7923 is connected
to a V
processor. The V
3 V supply of the microprocessor to allow a 3 V logic interface
(see the Digital Inputs section). The conversion result is output in
a 16-bit word. This 16-bit data stream consists of two leading zeros,
IN
Figure 7. Straight Binary Transfer Characteristic
DD
± REF
IN
/4096 for the AD7923. The ideal transfer characteristic
of 5 V, the serial interface is connected to a 3 V micro-
111…111
111…110
111…000
011…111
000…010
000…001
000…000
IN
011…111
011…110
000…001
000…000
111…111
100…010
100…001
100…000
•
•
•
•
•
Input Range
NOTE: V
•
•
•
•
DRIVE
IN
IN
0V
–V
and twos complement output coding is
becomes the zero code point, –REF
REF
REF
pin of the AD7923 is connected to the same
1 LSB
IS EITHER REF
1LSB
IN
IN
V
.
ANALOG INPUT
REF
ANALOG INPUT
becomes positive full scale, with
1LSB
IN
1LSB
1LSB
is connected to a decoupled
IN
OR 2
+V
V
REF
REF
2
+V
/4096
REF
REF
V
REF
1LSB
IN
4096
1 LSB
REV. 0
IN
IN
is