AD7322 Analog Devices, AD7322 Datasheet - Page 17

• Manufacturer Part Number: AD7322
• Description: Software Selectable True Bipolar Input, 2-Channel, 12-Bit Plus Sign ADC
• Manufacturer: Analog Devices

Specifications of AD7322

• Resolution (bits): 13bit
• # Chan: 2
• Sample Rate: 1MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni,SE-Uni
• Ain Range: Bip (Vref),Bip (Vref) x 2,Bip (Vref) x 4,Bip 10V,Bip 2.5V,Bip 5.0V,Uni (Vref) x 4,Uni 10V
• Adc Architecture: SAR
• Pkg Type: SOP

Care should be taken to ensure that the analog input does not
exceed the V
Exceeding this value causes the diodes to become forward
biased and to start conducting into either the V
V
causing irreversible damage to the part.
In Figure 29 and Figure 30, Capacitor C1 is typically 4 pF and
can primarily be attributed to pin capacitance. Resistor R1 is a
lumped component made up of the on resistance of the input
multiplexer and the track-and-hold switch. Capacitor C2 is the
sampling capacitor; its capacitance varies depending on the
analog input range selected (see the Specifications section).
Track-and-Hold Section
The track-and-hold on the analog input of the AD7322 allows
the ADC to accurately convert an input sine wave of full-scale
amplitude to 13-bit accuracy. The input bandwidth of the track-
and-hold is greater than the Nyquist rate of the ADC. The
AD7322 can handle frequencies up to 22 MHz.
The track-and-hold enters its tracking mode on the 14
rising edge after the CS falling edge. The time required to acquire
an input signal depends on how quickly the sampling capacitor
is charged. With zero source impedance, 305 ns is sufficient to
acquire the signal to the 13-bit level. The acquisition time
required is calculated using the following formula:
where C is the sampling capacitance and R is the resistance seen
by the track-and-hold amplifier looking back on the input.
For the AD7322, the value of R includes the on resistance of the
input multiplexer and is typically 300 Ω. R
any extra source impedance on the analog input.
SS
supply rail. These diodes can conduct up to 10 mA without
t
ACQ
= 10 × ((R
Figure 30. Equivalent Analog Input Circuit (Differential)
V
V
NOTES
1. V
IN
IN
DD
+
–
IN
+ REFERS TO V
and V
SOURCE
SS
C1
C1
supply rails by more than 300 mV.
+ R) C)
V
V
V
V
DD
DD
SS
SS
IN
D
D
D
D
0 AND V
IN
– REFERS TO V
SOURCE
R1
R1
DD
C2
C2
should include
IN
1.
supply rail or
th
SCLK
Rev. A | Page 17 of 36
The AD7322 enters track mode on the 14
When running the AD7322 at a throughput rate of 1 MSPS with
a 20 MHz SCLK signal, the ADC has approximately
to acquire the analog input signal. The ADC goes back into
hold mode on the CS falling edge.
As the V
the input multiplexer increases. Therefore, based on the equation
for t
time provided to the AD7322 and, therefore, decrease the
overall throughput rate. Figure 31 shows that if the throughput
rate is reduced when operating with minimum V
supplies, the specified THD performance is maintained.
Unlike other bipolar ADCs, the AD7322 does not have a
resistive analog input structure. On the AD7322, the bipolar
analog signal is sampled directly onto the sampling capacitor.
This gives the AD7322 high analog input impedance. An
approximation for the analog input impedance can be
calculated from the following formula:
where f
capacitor value.
C
Specifications section). When operating at 1 MSPS, the analog
input impedance is typically 75 kΩ for the ±10 V range. As the
sampling frequency is reduced, the analog input impedance
further increases. As the analog input impedance increases, the
current required to drive the analog input therefore decreases.
Figure 31. THD vs. ±V
S
depends on the analog input range chosen (see the
ACQ
1.5 SCLK + t
Z = 1/(f
–50
–55
–60
–65
–70
–75
–80
–85
–90
–95
, it is necessary to increase the amount of acquisition
S
is the sampling frequency, and C
DD
5
/V
1MSPS
S
SS
× C
supply voltage is reduced, the on resistance of
7
S
8
DD
)
750kSPS
+ t
/V
SS
QUIET
9
Supply Voltage at 500 kSPS, 750 kSPS, and 1 MSPS
±V
DD
/V
11
SS
SUPPLIES (V)
13
V
INTERNAL REFERENCE
T
f
±5V RANGE
SE MODE
IN
CC
A
= 25°C
= 10kHz
S
th
= V
is the sampling
15
SCLK rising edge.
DRIVE
500kSPS
= 5V
DD
17
and V
AD7322
1
9
SS