參數(shù)資料
型號: AD7609BSTZ
廠商: ANALOG DEVICES INC
元件分類: ADC
英文描述: 8-CH 18-BIT SUCCESSIVE APPROXIMATION ADC, SERIAL/PARALLEL ACCESS, PQFP64
封裝: ROHS COMPLIANT, MS-026BCD, LQFP-64
文件頁數(shù): 15/35頁
文件大?。?/td> 1737K
代理商: AD7609BSTZ
AD7609
Preliminary Technical Data
Rev. PrD | Page 22 of 35
THEORY OF OPERATION
CONVERTER DETAILS
The AD7609 is a data acquisition system that employs a high
speed, low power, charge redistribution successive approxima-
tion analog-to-digital converter (ADC) and allows the
simultaneous sampling of eight true differential analog input
channels. The analog inputs on the AD7609 can accept true
bipolar input signals. The RANGE pin is used to select either
±10 V or ±5 V as the input range. The AD7609 operates from
a single 5 V supply.
The AD7609 contains input clamp protection, input signal
scaling amplifiers, second-order antialiasing filter, track-and-
hold amplifiers, an on-chip reference, reference buffers, a high
speed ADC, digital filter and high speed parallel and serial
interfaces. Sampling on the AD7609 is controlled using
CONVST x signals.
ANALOG INPUT
Analog Input Ranges
The AD7609 can handle true bipolar input voltages. The logic
level on the RANGE pin determines the analog input range of
all analog input channels. If this pin is tied to a logic high, the
analog input range is ±10 V for all channels. If this pin is tied
to a logic low, the analog input range is ±5 V for all channels.
A logic change on this pin has an immediate effect on the
analog input range; however, there will be a settling time of
80 μs typically, in addition to the normal acquisition time
requirement. The recommended practice is to hardwire the
RANGE pin according to the desired input range for the system
signals.
Analog Input Impedance
The analog input impedance of the AD7609 is 1 MΩ. This is a
fixed input impedance and does not vary with the AD7609 sam-
pling frequency. This high analog input impedance eliminates
the need for a driver amplifier in front of the AD7609 allowing
for direct connection to the source or sensor. With the need for
a driver amplifier eliminated, bipolar supplies can be removed
from the signal chain, which are often a source of noise in a
system.
Analog Input Clamp Protection
Figure 33 shows the analog input structure of the AD7609.
Each AD7609 analog input contains clamp protection circuitry.
Despite a single 5 V supply operation, this analog input clamp
protection allows for an input over voltage up to ±16.5 V.
1M
CLAMP
Vx+
1M
CLAMP
Vx–
SECOND-
ORDER
LPF
RFB
09
76
0
-12
9
Figure 33. Analog Input Circuitry
Figure 34 shows the voltage vs. current characteristic of the
clamp circuit. For input voltages up to ±16.5 V, no current flows
in the clamp circuit. For input voltages above ±16.5 V the
AD7609 clamp circuitry turns on and clamps the analog input
to ±16.5 V. A series resister should be placed on the analog
input channels to limit the current to ±10 mA for input voltages
above ±16.5 V. In an application where there is a series
resistance on an analog input channel VINx+, a corresponding
resistance is required on the VINx channel, see Figure 35. If
there is no corresponding resister on the Vx channel, this will
result in a gain error on that channel.
30
–50
–40
–30
–20
–10
0
10
20
–20
–15
–10
–5
0
5
10
15
20
IN
P
UT
C
L
AM
P
CUR
RE
NT
(
m
A)
SOURCE VOLTAGE (V)
08
47
9-
0
33
AVCC, VDRIVE = 5V
TA = 25°C
Figure 34. Input Protection Clamp Profile
1M
CLAMP
VINx+
1M
CLAMP
VINx–
RFB
C
R
+10V
–10V
AD7609
09
76
0-
0
31
+10V
–10V
Figure 35. Input Resistance Matching on the Analog Input
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