Datasheet

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APPLICATION INFORMATION
RESISTOR MATCHING
ǒ
V
DD
Ǔ
)
ǒ
V
SS
Ǔ
2
(1)
_
+
x1
Output Buffer
Vcm Error
Amplifier
C R
C
R
x1
Output Buffer
V
OUT+
V
OUT-
V
DD
V
SS
V
IN-
V
IN+
30 kΩ
30 kΩ
V
DD
V
SS
V
OCM
THS4120
THS4121
SLOS319D – FEBRUARY 2001 – REVISED OCTOBER 2004
Resistor matching is important in fully differential amplifiers. The balance of the output on the reference voltage
depends on matched ratios of the resistors. CMRR, PSRR, and cancellation of the second harmonic distortion
diminish if resistor mismatch occurs. Therefore, it is recommended to use 1% tolerance resistors or better to
keep the performance optimized.
V
OCM
sets the dc level of the output signals. If no voltage is applied to the V
OCM
pin, it is set to the midrail voltage
internally defined as:
In the differential mode, the V
OCM
on the two outputs cancel each other. Therefore, the output in the differential
mode is the same as the input with the gain of 1. V
OCM
has a high bandwidth capability up to the typical
operation range of the amplifier. For the prevention of noise going through the device, use a 0.1- µ F capacitor on
the V
OCM
pin as a bypass capacitor. The following graph shows the simplified diagram of the THS412x.
Figure 18. THS412x Simplified Diagram
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