AD7608 Analog Devices, AD7608 Datasheet - Page 19

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AD7608

Manufacturer Part Number
AD7608
Description
8-Channel DAS with 18-Bit, Bipolar, Simultaneous Sampling ADC
Manufacturer
Analog Devices
Datasheet

Specifications of AD7608

Resolution (bits)
18bit
# Chan
8
Sample Rate
200kSPS
Interface
Par,Ser,SPI
Analog Input Type
Diff-Bip
Ain Range
Bip 10V,Bip 5.0V
Adc Architecture
SAR
Pkg Type
QFP

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Data Sheet
THEORY OF OPERATION
CONVERTER DETAILS
The AD7608 is a data acquisition system that employs a high
speed, low power, charge redistribution, successive approxi-
mation analog-to-digital converter (ADC) and allows the
simultaneous sampling of eight analog input channels. The
analog inputs on the AD7608 can accept true bipolar input
signals. The RANGE pin is used to select either ±10 V or
±5 V as the input range. The AD7608 operates from a single
5 V supply.
The AD7608 contains input clamp protection, input signal
scaling amplifiers, a second-order antialiasing filter, track-and-
hold amplifiers, an on-chip reference, reference buffers, a high
speed ADC, a digital filter, and high speed parallel and serial
interfaces. Sampling on the AD7608 is controlled using the
CONVST x signals.
ANALOG INPUT
Analog Input Ranges
The AD7608 can handle true bipolar, single-ended 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 the RANGE pin has an immediate
effect on the analog input range; how-ever, there is typically
a settling time of approximately 80 µs, 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.
During normal operation, the applied analog input voltage
should remain within the analog input range selected via the
range pin. A RESET pulse must be applied after power-up to
ensure the analog input channels are configured for the range
selected.
When in a power-down mode, it is recommended to tie the
analog inputs to GND. As per the input clamp protection section,
the overvoltage clamp protection is recommended for use in
transient overvoltage conditions and should not remain active
for extended periods. Stressing the analog inputs outside of the
conditions mentioned here may degrade the Bipolar Zero Code
error and THD performance of the AD7608.
Analog Input Impedance
The analog input impedance of the AD7608 is 1 MΩ. This is
a fixed input impedance that does not vary with the AD7608
sampling frequency. This high analog input impedance elimi-
nates the need for a driver amplifier in front of the AD7608,
allowing for direct connection to the source or sensor. With
the need for a driver amplifier eliminated, bipolar supplies
(which are often a source of noise in a system) can be
removed from the signal chain.
Rev. A | Page 19 of 32
Analog Input Clamp Protection
Figure 31 shows the analog input structure of the AD7608. Each
AD7608 analog input contains clamp protection circuitry. Despite
single 5 V supply operation, this analog input clamp protection
allows for an input overvoltage up to ±16.5 V.
Figure 32 shows the voltage vs. current characteristic of the
clamp circuit. For input voltages of up to ±16.5 V, no current
flows in the clamp circuit. For input voltages that are above
±16.5 V, the AD7608 clamp circuitry turns on.
A series resistor 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, Vx, a corresponding resistance is required on the
analog input GND channel, VxGND (see Figure 33). If there is
no corresponding resistor on the VxGND channel, an offset
error occurs on that channel.
VxGND
ANALOG
SIGNAL
–10
–20
–30
–40
INPUT
30
20
10
Vx
Figure 33. Input Resistance Matching on the Analog Input
0
–25
–20
CLAMP
CLAMP
Figure 32. Input Protection Clamp Profile
R
R
–15
C
Figure 31. Analog Input Circuitry
AV
T
1MΩ
1MΩ
VxGND
–10
A
CC
= 25 °C
SOURCE VOLTAGE (V)
Vx
, V
–5
DRIVE
R
R
FB
FB
AD7608
CLAMP
CLAMP
= 5V
0
SECOND-
ORDER
5
LPF
1MΩ
1MΩ
10
15
R
R
FB
FB
AD7608
20
25

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