AD7606/AD7606-6/AD7606-4
THEORY OF OPERATION
Analog Input Clamp Protection
CONVERTER DETAIꢀS
Figure 34 shows the analog input structure of the AD7606/
AD7606-6/AD7606-4. Each analog input of the AD7606/
AD7606-6/AD7606-4 contains clamp protection circuitry.
Despite single 5 V supply operation, this analog input clamp
protection allows for an input over voltage of up to 16.5 V.
The AD7606/AD7606-6/AD7606-4 are data acquisition systems
that employ a high speed, low power, charge redistribution,
successive approximation analog-to-digital converter (ADC)
and allow the simultaneous sampling of eight/six/four analog input
channels. The analog inputs on the AD7606/AD7606-6/AD7606-4
can accept true bipolar input signals. The RANGE pin is used to
select either 10 V or 5 V as the input range. The AD7606/
AD7606-6/AD7606-4 operate from a single 5 V supply.
R
FB
1MΩ
Vx
CLAMP
CLAMP
1MΩ
VxGND
The AD7606/AD7606-6/AD7606-4 contain input clamp
protection, input signal scaling amplifiers, a second-order anti-
aliasing 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 AD7606/
AD7606-6/AD7606-4 is controlled using the CONVST signals.
SECOND-
ORDER
LPF
R
FB
Figure 34. Analog Input Circuitry
Figure 35 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 AD7606/AD7606-6/AD7606-4 clamp circuitry turns on.
ANAꢀOG INPUT
Analog Input Ranges
AV , V
CC DRIVE
= 5V
30
20
T
= 25°C
A
The AD7606/AD7606-6/AD7606-4 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 this pin has an
immediate effect on the analog input range; however, 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.
10
0
–10
–20
–30
–40
–50
–20
–15
–10
–5
0
5
10
15
20
Analog Input Impedance
SOURCE VOLTAGE (V)
The analog input impedance of the AD7606/AD7606-6/
AD7606-4 is 1 MΩ. This is a fixed input impedance that does
not vary with the AD7606 sampling frequency. This high analog
input impedance eliminates the need for a driver amplifier in
front of the AD7606/AD7606-6/AD7606-4, 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.
Figure 35. Input Protection Clamp Profile
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 36). If there is
no corresponding resistor on the VxGND channel, an offset
error occurs on that channel.
R
FB
AD7606
ANALOG
INPUT
SIGNAL
R
R
1MΩ
1MΩ
Vx
CLAMP
CLAMP
C
VxGND
R
FB
Figure 36. Input Resistance Matching on the Analog Input of the
AD7606/AD7606-6/AD7606-4
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