THS4120 Datasheet, PDF (11/23 Pages) Texas Instruments – HIGH-SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS

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THS4120
THS4121
SLOS319D – FEBRUARY 2001 – REVISED OCTOBER 2004
DATA CONVERTERS
Data converters are one of the most popular applications for the fully differential amplifiers.
Fully differential amplifiers can operate with a single supply. V_OCM defaults to the midrail voltage, V_DD/2. The
differential output may be fed into a data converter. This method eliminates the use of a transformer in the circuit.
If the ADC has a reference voltage output (V_ref), then it is recommended to connect it directly to the V_OCM of the
amplifier using a bypass capacitor for stability. For proper operation, the input common-mode voltage to the input
terminal of the amplifier should not exceed the common-mode input voltage range.
0.1 µF
V_DD
3.3 V
V_IN
+-
V_OCM
-+
3.3 V
AV_DD DV_DD
A_IN1
A_{IN2 AVSS}
V_ref
Figure 19. Differential Amplifier Using a Single Supply
Some single-supply applications may require the input voltage to exceed the common-mode input voltage range.
In such cases, the following circuit configuration is suggested to bring the common-mode input voltage within the
specifications of the amplifier.
0.1 µF
V_CC
R_f V_DD
R_PU
R_(g)
V_IN
+
V_{P VOCM}
3.3 V
-
R_(g)
R_PU
V_CC
-+
R_f
V_DD
V_OUT
AV_DD
A_IN1
DV_DD
V_OUT ^{AIN2 AVSS}
V_ref
Figure 20. Circuit With Improved Common-Mode Input Voltage
The following equation is used to calculate R_PU:
ǒ Ǔ ǒ Ǔ ^{RPU +}
VP – VDD
VIN – VP
1
R(g)
)
VOUT – VP
1
Rf
(2)
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