AD7922 Analog Devices, AD7922 Datasheet - Page 16

• Manufacturer Part Number: AD7922
• Description: 2-Channel, 2.35 V to 5.25 V, 1 MSPS, 12-Bit A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7922

• Resolution (bits): 12bit
• # Chan: 2
• Sample Rate: 1MSPS
• Interface: Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: Uni Vdd
• Adc Architecture: SAR
• Pkg Type: SOP,SOT

AD7912/AD7922
TYPICAL CONNECTION DIAGRAM
Figure 22 shows a typical connection diagram for the AD7912/
AD7922. V
should be well decoupled. This provides an analog input range
of 0 V to V
with two leading zeros, followed by the channel identifier bit
that identifies the channel converted, followed by the mode bit
that indicates the current mode of operation, and by the MSB of
the 12-bit or 10-bit result. For the AD7912, the 10-bit result is
followed by two trailing zeros. See the Serial Interface section.
Alternatively, because the supply current required by the
AD7912/AD7922 is so low, a precision reference can be used as
the supply source to the AD7912/AD7922. A REF19x voltage
reference (REF195 for 5 V or REF193 for 3 V) can be used to
supply the required voltage to the ADC (see Figure 22). This
configuration is especially useful, if the power supply is quite
noisy or if the system supply voltages are at some value other
than 5 V or 3 V (for example, 15 V). The REF19x outputs a
steady voltage to the AD7912/AD7922. If the low dropout
REF193 is used, the current it needs to supply to the AD7912/
AD7922 is typically1.5 mA. When the ADC is converting at a
rate of 1 MSPS, the REF193 needs to supply a maximum of
2 mA to the AD7912/AD7922. The load regulation of the
REF193 is typically 10 ppm/mA (REF193, VS = 5 V), which
results in an error of 20 ppm (60 µV) for the 2 mA drawn from
it. This corresponds to a 0.082 LSB error for the AD7922 with
V
AD7912.
For applications where power consumption is a concern, the
power-down mode of the ADC and the sleep mode of the
REF19x reference should be used to improve power perform-
ance. See the Modes of Operation section.
DD
0V TO V
680nF
= 3 V from the REF193 and a 0.061 LSB error for the
INPUT
1.5mA
DD
Figure 22. REF193 as Power Supply to AD7912/AD7922
DD
REF
. The conversion result is output in a 16-bit word
V
V
V
GND
is taken internally from V
DD
IN0
IN1
0.1
µ
AD7912/
AD7922
F
TANT
1
3V
µ
F
DOUT
SCLK
INTERFACE
DIN
CS
SERIAL
REF193
10
µ
DD
F
and as such V
µ
0.1
C/
µ
µ
P
F
5V
SUPPLY
DD
Rev. 0 | Page 16 of 32
Table 6 provides some typical performance data with various
references used as a V
the same setup conditions.
Table 6. AD7922 Performance for Various Voltage
References IC
Reference Tied to V
AD780 at 3 V
REF193
ADR433
AD780 at 2.5 V
REF192
ADR421
ANALOG INPUT
Figure 23 shows an equivalent circuit of the analog input
structure of the AD7912/AD7922. The two diodes, D1 and D2,
provide ESD protection for the analog input. Care must be
taken to ensure that the analog input signal never exceeds the
supply rails by more than 300 mV, because this would cause
these diodes to become forward biased and start conducting
current into the substrate. The maximum current these diodes
can conduct without causing irreversible damage to the part is
10 mA.
The capacitor C1 in Figure 23 is typically about 6 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. This resistor is typically about 100 Ω.
The capacitor C2 is the ADC sampling capacitor and has a
capacitance of 20 pF typically.
For ac applications, removing high frequency components from
the analog input signal is recommended using a band-pass filter
on the relevant analog input pin. In applications where har-
monic distortion and signal-to-noise ratio are critical, the
analog input should be driven from a low impedance source.
Large source impedances can significantly affect the ac
performance of the ADC. This might necessitate the use of an
input buffer amplifier. The choice of the op amp is a function of
the particular application.
V
IN
Figure 23. Equivalent Analog Input Circuit
6pF
C1
DD
DD
source and a 50 kHz input tone under
V
DD
AD7922 SNR Performance (dB)
−73
−72.42
−72.9
−72.86
−72.27
−72.75
D1
D2
R1
20pF
C2