Datasheet

AD8224 Data Sheet
Rev. C | Page 24 of 28
APPLICATIONS INFORMATION
DRIVING AN ADC
An instrumentation amplifier is often used in front of an ADC
to provide CMRR and additional conditioning such as a voltage
level shift and gain (see Figure 62). In this example, a 2.7 nF
capacitor and a 500 Ω resistor create an antialiasing filter for the
AD7685. The 2.7 nF capacitor also serves to store and deliver
the necessary charge to the switched capacitor input of the ADC.
The 500 Ω series resistor reduces the burden of the 2.7 nF load
from the amplifier. However, large source impedance in front of
the ADC can degrade the total harmonic distortion (THD).
For applications where THD performance is critical, the series
resistor needs to be small. At worst, a small series resistor can
load the AD8224, potentially causing the output to overshoot
or ring. In such cases, a buffer amplifier, such as the AD8615
should be used after the AD8224 to drive the ADC.
AD8224
AD7685
4.7µF
ADR435
+5V
2.7nF
REF
500Ω
1.07kΩ
+2.5V
+IN
–IN
±50mV
+5V
0.1µF10µF
06286-063
+
Figure 62. Driving an ADC in a Low Frequency Application
DIFFERENTIAL OUTPUT
The differential configuration of the AD8224 has the same
excellent dc precision specifications as the single-ended output
configuration and is recommended for applications in the
frequency range of dc to 1 MHz.
The circuit configuration, outlined in Table 4 and Table 7, refers
to the configuration shown in Figure 63 only. The circuit includes
an RC filter that maintains the stability of the loop.
The transfer function for the differential output is
V
DIFF_OUT
= V
+OUT
− V
−OUT
= (V
+IN
− V
−IN
) × G
where:
GR
G
kΩ49.4
1 +=
+IN
–IN
+
–
+
–
AD8224
AD8224
+OUT
33pF
–OUT
+IN2
REF2
20kΩ
R
G
06286-064
Figure 63. Differential Circuit Schematic
Setting the Common-Mode Voltage
The output common-mode voltage is set by the average of +IN2
and REF2. The transfer function is
V
CM_OUT
= (V
+OUT
+ V
−OUT
)/2 = (V
+IN2
+ V
REF2
)/2
+IN2 and REF2 have different properties that allow the
reference voltage to be easily set for a wide variety of applications.
+IN2 has high impedance but cannot swing to the positive
supply rail. REF2 must be driven with a low impedance but
can go 300 mV beyond the supply rails.
A common application sets the common-mode output voltage
to the midscale of a differential ADC. In this case, the ADC
reference voltage is sent to the +IN2 terminal, and ground is
connected to the REF2 terminal. This produces a common-
mode output voltage of half the ADC reference voltage.
2-Channel Differential Output Using a Dual Op Amp
Another differential output topology is shown in Figure 64.
Instead of a second in-amp, ½ of a dual OP2177 op amp creates
the inverted output. Because the OP2177 comes in an MSOP,
this configuration allows the creation of a dual-channel, precision
differential output in-amp with little board area.
Errors from the op amp are common to both outputs and are,
thus, common mode. Errors from mismatched resistors also
create a common-mode dc offset. Because these errors are
common mode, they are likely to be rejected by the next
device in the signal chain.
+IN
–IN
REF
AD8224
V
REF
4.99kΩ
+
–
OP2177
+OUT
–OUT
4.99kΩ
06286-065
Figure 64. Differential Output Using Op Amp