AD7922 Analog Devices, AD7922 Datasheet - Page 17

• 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

Table 7 provides some typical performance data with various
op amps used as the input buffer, and a 50 kHz input tone under
the same setup conditions.
Table 7. AD7922 Performance for Various Input Buffers
Op Amp in the Input
Buffer
Single op amps
Dual op amps
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance depends on the amount of total harmonic
distortion (THD) that can be tolerated. The THD increases as
the source impedance increases and performance degrades (see
Figure 16).
DIGITAL INPUTS
The digital inputs applied to the AD7912/AD7922 are not
limited by the maximum ratings that limit the analog input.
Instead, the digital inputs applied can go to 7 V and are not
restricted by the V
example, if the AD7912/AD7922 are operated with a V
then 5 V logic levels could be used on the digital inputs. How-
ever, it is important to note that the data output on DOUT still
has 3 V logic levels when V
SCLK, DIN, and CS not being restricted by the V
is that power supply sequencing issues are avoided. If CS , DIN,
or SCLK are applied before V
as there would be on the analog inputs if a signal greater than
0.3 V were applied prior to V
DIN INPUT
The channel to be converted on in the next conversion is
selected by writing to the DIN pin. Data on the DIN pin is
loaded into the AD7912/AD7922 on the falling edge of SCLK.
The data is transferred into the part on the DIN pin at the same
time that the conversion result is read from the part. Only the
third and fourth bits of the DIN word are used; the rest are
ignored by the ADC.
The third MSB is the channel identifier bit, which identifies the
channel to be converted on in the next conversion,
V
IN0
AD8038
AD8510
AD8021
AD712
AD8022
(CHN = 0) or V
DD
IN1
+ 0.3 V limit as on the analog input. For
(CHN = 1).
DD
DD
DD
= 3 V. Another advantage of
AD7922 SNR Performance (dB)
50 kHz Input , V
−72.79
−72.35
−72.2
−72.68
−72.88
.
, then there is no risk of latch-up
DD
DD
= 3.6 V
+ 0.3 V limit
DD
of 3 V,
Rev. 0 | Page 17 of 32
The fourth MSB, STY, is related to the mode of operation of the
device. To keep the AD7912/ AD7922 in daisy-chain mode, the
CHN and STY bits have to be inverted during the conversions
(STY ≠ CHN). A conversion with STY = CHN on the input
forces the device to normal mode in the next cycle. See the
Daisy-Chain Mode section for more details.
If the AD7912/AD7922 are not going to be used in daisy-chain
mode, it is recommended to keep STY and CHN the same
(STY = CHN). In that case, the channel can be selected by tying
DIN either high or low during a conversion cycle.
To summarize:
CHN = 0 , Channel 0 selected for next conversion.
CHN = 1 , Channel 1 selected for next conversion.
CHN = STY , forces normal mode in the next cycle.
CHN ≠ STY , keeps the AD7912/AD7922 in daisy-chain mode.
DOUT OUTPUT
The conversion result from the AD7912/AD7922 is provided on
this output as a serial data stream. The bits are clocked out on
the SCLK falling edge at the same time that the conversion is
taking place.
The serial data stream for the AD7922 consists of two leading
zeros followed by the bit that identifies the channel converted,
the bit that indicates the current mode of operation, and the
12-bit conversion result with MSB provided first.
For the AD7912, the serial data stream consists of two leading
zeros followed by the bit that identifies the channel converted,
the bit that indicates the current mode of operation, and the
10-bit conversion result with MSB provided first, followed by
two trailing zeros.
The CHN and MOD bits on DOUT indicate to the user the
current mode of operation of the ADC. If CHN = MOD,
the AD7912/AD7922 are in normal mode. Otherwise, if
CHN ≠ MOD, the AD7912/AD7922 are in daisy-chain mode.
MSB
0
0
MSB
X
0
0
CHN MOD CONVERSION RESULT
CHN MOD
X
Figure 25. AD7912/AD7922 DOUT Word
Figure 24. AD7912/AD7922 DIN Word
CHN
STY
CONVERSION RESULT
DON'T CARE
AD7912/AD7922
0
LSB
0
LSB
AD7912
AD7922