Datasheet

Data Sheet AD7176-2
Rev. A | Page 23 of 68
CIRCUIT DESCRIPTION
ANALOG INPUT
The AD7176-2 has five analog input pins: AIN0, AIN1, AIN2,
AIN3, and AIN4. Each of these pins connects to the internal
crosspoint multiplexer. The crosspoint multiplexer enables any of
these inputs to be configured as an input pair, either pseudo
differential or fully differential. The AD7176-2 can have up to four
active channels. When more than one channel is enabled, the
channels are automatically sequenced in order. The output of
the multiplexer is connected directly to the switched-capacitor
input of the ADC. The simplified analog input circuit is shown in
Figure 29.
Figure 29. Simplified Analog Input Circuit
The CS1 and CS2 capacitors have a magnitude in the order of a
number of picofarads each. This capacitance is the combination
of both the sampling capacitance and the parasitic capacitance.
The average input current to the AD7176-2 changes linearly with
the differential input voltage at a rate of 48 μA/V. Each of the
analog inputs must be buffered externally not only to provide
the varying input current with differential input amplitude but
also to settle the switched-capacitor input to allow for accurate
sampling.
Recommended amplifiers for this purpose are discussed in the
Driver Amplifiers section.
Fully Differential Inputs
Because the AIN0 to AIN4 analog inputs are connected to a
crosspoint multiplexer, any combination of signals can be used
to create an analog input pair. This allows the user to select two
fully differential inputs or four pseudo differential inputs.
If two fully differential input paths are connected to the AD7176-2,
using AIN0/AIN1 as one differential input pair and AIN2/AIN3
as the second differential input pair is recommended. This is
due to the relative locations of these pins to each other. All
analog inputs should be decoupled to AVSS.
Pseudo Differential Inputs
The user can also choose to measure four different single-ended
analog inputs. In this case, each of the analog inputs is converted
as being the difference between the single-ended input to be
measured and a set analog input common pin. Because there is
a crosspoint multiplexer, the user can set any of the analog inputs
as the common pin. An example of such a scenario is to connect
the AIN4 pin to AVSS or to the REFOUT voltage (that is, AVSS
+ 2.5 V) and select this input when configuring the crosspoint
multiplexer. When using the AD7176-2 with pseudo differential
inputs, the INL specification is degraded.
DRIVER AMPLIFIERS
To drive the analog input switch capacitor, an external amplifier
is required. Details of three recommended amplifiers for the
AD7176-2 are shown in the Driver Amplifiers section. Each of
the amplifiers can run from a single 5 V voltage rail.
A
IN0
A
IN1
A
V
DD1
AVSS
AVSS
AVSS
A VDD1
AVSS
A
IN2
AVDD1
A
IN4
AVDD1
A
IN3
A VDD1
AVSS
Ø1
CS1
CS2
+IN
–IN
Ø2
Ø2
Ø1
11037-056