AD7450A_15 Datasheet, PDF (19/29 Pages) Analog Devices – Differential Input, 1 MSPS 10-Bit and 12-Bit ADCs in an 8-Lead SOT-23

AD7440/AD7450A
DRIVING DIFFERENTIAL INPUTS
Differential operation requires VIN+ and VIN– to be driven
simultaneously with two equal signals that are 180° out of
phase. The common mode must be set up externally and has a
range determined by VREF, the power supply, and the particular
amplifier used to drive the analog inputs (see Figure 28 and
Figure 29). Differential modes of operation with either an ac or
dc input provide the best THD performance over a wide
frequency range. Because not all applications have a signal
preconditioned for differential operation, there is often a need
to perform single-ended-to-differential conversion.
Differential Amplifier
An ideal method of applying differential drive to the
AD7440/AD7450A is to use a differential amplifier such as the
AD8138. This part can be used as a single-ended-to-differential
amplifier or as a differential-to-differential amplifier. In both
cases, the analog input needs to be bipolar. It also provides
common-mode level shifting and buffering of the bipolar input
signal. Figure 34 shows how the AD8138 can be used as a
single-ended-to-differential amplifier. The positive and negative
outputs of the AD8138 are connected to the respective inputs
on the ADC via a pair of series resistors to minimize the effects
of switched capacitance on the front end of the ADCs. The RC
low-pass filter on each analog input is recommended in ac
applications to remove high frequency components of the
analog input. The architecture of the AD8138 results in outputs
that are very highly balanced over a wide frequency range
without requiring tightly matched external components.
If the analog input source being used has zero impedance, all
four resistors (RG1, RG2, RF1, and RF2) should be the same. If
the source has a 50 Ω impedance and a 50 Ω termination, for
example, the value of RG2 should be increased by 25 Ω to
balance this parallel impedance on the input and thus ensure
that both the positive and negative analog inputs have the same
gain (see Figure 34). The outputs of the amplifier are perfectly
matched, balanced differential outputs of identical amplitude
and are exactly 180° out of phase.
The AD8138 is specified with +3 V, +5 V, and ±5 V power
supplies, but the best results are obtained with a ±5 V supply.
The AD8132 is a lower cost device that could also be used in
this configuration with slight differences in characteristics to
the AD8138 but with similar performance and operation.
+2.5V
GND
–2.5V
R_F1
R_S*
R_G1
51Ω
V_{OCM AD8138}
R_G2
R_S*
C*
C*
R_F2
3.75V
2.5V
1.25V
V_IN+
AD7440/
AD7450A
V_IN–
V_REF
3.75V
2.5V
1.25V
*MOUNT AS CLOSE TO THE AD7440/AD7450A AS POSSIBLE
AND ENSURE HIGH PRECISION R_S AND C_S ARE USED.
R_S–50Ω; C–1nF
R_G1 = R_F1 = R_F2 = 499Ω; R_G2 = 523Ω
EXTERNAL
V_REF (2.5V)
Figure 34. Using the AD8138 as a Single-Ended-to-Differential Amplifier
Rev. C | Page 18 of 28