Datasheet

AD7693
Rev. A | Page 15 of 24
Table 8. Recommended Driver Amplifiers
Amplifier Typical Application
ADA4941-1 Very low noise, low power single to differential
ADA4841-x Very low noise, small, and low power
AD8655 5 V single supply, low noise
AD8021 Very low noise and high frequency
AD8022 Low noise and high frequency
OP184 Low power, low noise, and low frequency
AD8605, AD8615 5 V single supply, low power
SINGLE-ENDED-TO-DIFFERENTIAL DRIVER
For applications using a single-ended analog signal, either
bipolar or unipolar, the
ADA4941-1 single-ended-to-differential
driver allows for a differential input into the part. The
schematic is shown in Figure 31.
R1 and R2 set the attenuation ratio between the input range and
the ADC range (V
REF
). R1, R2, and C
F
are chosen depending on
the desired input resistance, signal bandwidth, antialiasing and
noise contribution. For example, for the ±10 V range with a 4 kΩ
impedance, R2 = 1 kΩ and R1 = 4 kΩ.
R3 and R4 set the common mode on the IN− input, and R5 and
R6 set the common mode on the IN+ input of the ADC. The
common mode should be set close to V
REF
/2; however, if single
supply is desired, it can be set slightly above V
REF
/2 to provide
some headroom for the
ADA4941-1 output stage. For example,
for the ±10 V range with a single supply, R3 = 8.45 kΩ, R4 =
11.8 kΩ, R5 = 10.5 kΩ, and R6 = 9.76 kΩ.
AD7693
REF
GND
VDD
IN+
2.7nF
100nF
2.7nF
IN–
+5V REF
±
10V, ±5V, ...
+5.2V
+5.2V
33Ω
10µF
R2
C
F
ADA4941
R1
R3
100nF
R5
R4
R6
33Ω
06394-029
Figure 31. Single-Ended-to-Differential Driver Circuit
VOLTAGE REFERENCE INPUT
The AD7693 voltage reference input, REF, has a dynamic input
impedance and should therefore be driven by a low impedance
source with efficient decoupling between the REF and GND
pins, as explained in the Layout section.
When REF is driven by a very low impedance source (for
example, a reference buffer using the AD8031 or the AD8605),
a 10 μF (X5R, 0805 size) ceramic chip capacitor is appropriate
for optimum performance.
If an unbuffered reference voltage is used, the decoupling value
depends on the reference used. For instance, a 22 μF (X5R,
1206 size) ceramic chip capacitor is appropriate for optimum
performance using low temperature drift ADR43x and ADR44x
references.
If desired, smaller reference decoupling capacitor values down
to 2.2 μF can be used with a minimal impact on performance,
especially DNL.
Regardless, there is no need for an additional lower value
ceramic decoupling capacitor (for example, 100 nF) between the
REF and GND pins.
POWER SUPPLY
The AD7693 uses two power supply pins: a core supply, VDD,
and a digital input/output interface supply, VIO. VIO allows
direct interface with any logic between 1.8 V and VDD. To
reduce the supplies needed, the VIO and VDD pins can be tied
together. The AD7693 is independent of power supply sequencing
between VIO and VDD. Additionally, it is very insensitive to
power supply variations over a wide frequency range, as shown
in Figure 32.
50
55
60
65
70
75
80
85
90
95
100
1 10 100 1000 10000
FREQUENCY (kHz)
PSRR (dB)
06394-030
VREF = 5V
Figure 32. PSRR vs. Frequency
The AD7693 powers down automatically at the end of each
conversion phase; therefore, the operating currents and power
scale linearly with the sampling rate (refer to Figure 21). This
makes the part ideal for low sampling rates (even of a few hertz)
and low battery-powered applications.