AD7908BRUZ Analog Devices Inc, AD7908BRUZ Datasheet - Page 19

• Manufacturer Part Number: AD7908BRUZ
• Description: IC ADC 8BIT 8CH 1MSPS 20-TSSOP
• Manufacturer: Analog Devices Inc

Specifications of AD7908BRUZ

• Data Interface: DSP, MICROWIRE™, QSPI™, Serial, SPI™
• Number Of Bits: 8
• Sampling Rate (per Second): 1M
• Number Of Converters: 1
• Power Dissipation (max): 13.5mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP (0.173", 4.40mm Width)
• Resolution (bits): 8bit
• Sampling Rate: 1MSPS
• Input Channel Type: Single Ended
• Supply Voltage Range - Analog: 2.7V To 5.25V
• Supply Current: 2.7mA
• Digital Ic Case Style: TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-AD79X8CBZ - BOARD EVALUATION FOR AD79X8
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

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 can
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, 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 8).
ADC TRANSFER FUNCTION
The output coding of the AD7908/AD7918/AD7928 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
on). The LSB size is REF
the AD7918, and REF
characteristic for the AD7908/AD7918/AD7928 when straight
binary coding is selected is shown in Figure 17, and the ideal
transfer characteristic for the AD7908/AD7918/AD7928 when
twos complement coding is selected is shown in Figure 18.
V
IN
4pF
C1
111…111
111…110
111…000
011…111
000…010
000…001
000…000
Figure 17. Straight Binary Transfer Characteristic
•
•
•
•
•
Figure 16. Equivalent Analog Input Circuit
0V
NOTE
1. V
D1
D2
1 LSB
AV
IN
REF
DD
/4096 for the AD7928. The ideal transfer
IN
IS EITHER REF
CONVERSION PHASE: SWITCH OPEN
TRACK PHASE: SWITCH CLOSED
/256 for the AD7908, REF
ANALOG INPUT
1LSB = V
1LSB = V
1LSB = V
IN
OR 2 × REF
REF
REF
REF
+V
/256 AD7908
/1024 AD7918
/4096 AD7928
R1
REF
– 1 LSB
IN.
30pF
C2
IN
/1024 for
Rev. B | Page 19 of 28
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
TYPICAL CONNECTION DIAGRAM
Figure 20 shows a typical connection diagram for the
AD7908/AD7918/AD7928. In this setup, the AGND pin is
connected to the analog ground plane of the system. In Figure 20,
REF
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 AD7908/AD7918/AD7928 is connected to a V
of 5 V, the serial interface is connected to a 3 V microprocessor.
The V
to the same 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 a leading zero, three address bits indicating which
channel the conversion result corresponds to, followed by the 12
bits of conversion data for the AD7928 (10 bits of data for the
AD7918 and 8 bits of data for the AD7908, each followed by
two and four trailing zeros, respectively). For applications
where power consumption is of concern, the power-down
modes should be used between conversions or bursts of several
conversions to improve power performance (see the Modes of
Operation section).
IN
DRIVE
is connected to a decoupled 2.5 V supply from a reference
011…111
011…110
000…001
000…000
111…111
100…010
100…001
100…000
Figure 18. Twos Complement Transfer Characteristic
pin of the AD7908/AD7918/ AD7928 is connected
•
•
•
•
–V
REF
IN
IN
+ 1 LSB
with REF
and twos complement output coding is
becomes the zero code point, −REF
AD7908/AD7918/AD7928
ANALOG INPUT
IN
V
IN
REF
± REF
IN
.
becomes positive full scale, with
– 1 LSB
1LSB = 2 × V
1LSB = 2 × V
1LSB = 2 × V
IN
Input Range
+V
REF
REF
REF
REF
/256 AD7908
/1024 AD7918
/4096 AD7928
– 1 LSB
IN
is
IN
DD