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參數資料
| 型號： | AD7888ARU |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | +2.7 V to +5.25 V, Micropower, 8-Channel, 125 kSPS, 12-Bit ADC in 16-Lead TSSOP |
| 中文描述： | 8-CH 12-BIT SUCCESSIVE APPROXIMATION ADC, SERIAL ACCESS, PDSO16 |
| 封裝： | TSSOP-16 |
| 文件頁數： | 9/16頁 |
| 文件大小： | 164K |
| 代理商： | AD7888ARU |
REV. 0
AD7888
–9–
TYPICAL CONNECTION DIAGRAM
Figure 8 shows a typical connection diagram for the AD7888.
Both AGND pins are connected to the analog ground plane of
the system. V
REF
is connected to a well decoupled V
DD
pin to
provide an analog input range of 0 V to V
DD
. The conversion
result is output in a 16-bit word with four leading zeroes fol-
lowed by the MSB of the 12-bit result. For applications where
power consumption is of concern, the automatic power down at
the end of conversion should be used to improve power perfor-
mance. See Modes of Operation section of the data sheet.
DOUT
DIN
SCLK
CS
m
C/
m
P
AIN1
AIN2
AIN8
AGND
AGND
0.1
m
F
10
m
F
SUPPLY +2.7V
TO +5.25V
0V TO
REF IN/
REF OUT
INPUT
SERIAL
INTERFACE
REF IN/
REF OUT
AD7888
V
DD
Figure 8. Typical Connection Diagram
Analog Input
Figure 9 shows an equivalent circuit of the analog input structure
of the AD7888. The two diodes D1 and D2 provide ESD pro-
tection 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. 20 mA is
the maximum current these diodes can conduct without causing
irreversible damage to the part. However, it is worth noting that
a small amount of current (1 mA) being conducted into the
substrate due to an overvoltage on an unselected channel, can
cause inaccurate conversions on a selected channel. The capaci-
tor C1 in Figure 9 is typically about 4 pF and can primarily be
attributed to pin capacitance. The resistor R1 is a lumped com-
ponent made up of the on resistance of a multiplexer and a switch.
This resistor is typically about 100
. The capacitor C2 is the
ADC sampling capacitor and has a capacitance of 20 pF typically.
Note: The analog input capacitance seen when the track and
hold is in track mode is typically 38 pF, while in hold mode it is
typically 4 pF.
V
IN
V
DD
D2
R1
C1
4pF
CONVERSION PHASE – SWITCH OPEN
TRACK PHASE – SWITCH CLOSED
D1
C2
20pF
Figure 9. Equivalent Analog Input Circuit
For ac applications, removing high frequency components from
the analog input signal is recommended by use of 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.
Figure 10 shows a graph of the total harmonic distortion versus
analog input signal frequency for different source impedances.
INPUT FREQUENCY – kHz
–90
0.15
42.14
T
10.89
31.59
21.14
–85
–80
–75
–70
–65
49.86
THD vs. FREQUENCY FOR DIFFERENT
SOURCE IMPEDANCES
V
= +5V
5V EXT REFERENCE
R
IN
= 1k
V
, C
IN
= 100pF
R
IN
= 50
V
, C
IN
= 2.2nF
R
IN
= 10
V
, C
IN
= 10nF
Figure 10. THD vs. Analog Input Frequency
Analog Input Selection
On power-up, the default AIN selection is AIN1. When return-
ing to normal operation from power-down, the AIN selected
will be the same one that was selected prior to power-down
being initiated. Table II below shows the multiplexer address
corresponding to each analog input from AIN1 to AIN8 for the
AD7888.
Table II. Channel Configurations
ADD2
ADD1
ADD0
Analog Input Channel
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
AIN1
AIN2
AIN3
AIN4
AIN5
AIN6
AIN7
AIN8
On-Chip Reference
The AD7888 has an on-chip 2.5 V reference. This reference can
be enabled or disabled by clearing or setting the REF bit in the
Control Register, respectively. If the on-chip reference is to be
used externally in a system, it must be buffered before it is ap-
plied elsewhere. If an external reference is applied to the device,
the internal reference is automatically overdriven. However, in
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