Datasheet
SLOS310B − DECEMBER 2000 − REVISED SEPTEMBER 2006
11
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
gain setting resistors, R
F
and R
S
The differential gain for the TLV4120, with A1 configured as an inverter, is set by resistors R
F
and R
S
using the
following equation:
DiffGain + –2
ǒ
R
F
R
S
Ǔ
The differential gain for the TLV4120, with A1 in a noninverting configuration, is set by resistors R
F
and R
S
using
the following equation:
DiffGain + 2
ǒ1 )
R
F
R
S
Ǔ
Differential drive operation brings about the factor 2 in the gain equation due to amplifier A2, configured as an
inverter, mirroring the voltage swing across the load.
Given that the TLV4120 is a MOS amplifier, the input impedance is very high; consequently input bias currents
in most cases will not generally be a concern (see offset voltage application section). However, the noise in the
circuit will increase as R
F
increases. Typical values for R
F
will range between 5 kΩ and 20 kΩ.
Large values of feedback resistor, R
F
, at low gains can cause instability due to the pole caused by the input
capacitance. This can be alleviated by either reducing the size of R
F
or by putting a small capacitor in parallel
with R
F
.
AC coupling capacitor, C
S
When the input to the TLV4120 will be AC coupled to the input source, a high pass filter is formed with a corner
frequency equal to:
f
c
+
1
2pR
S
C
S
The value of C
S
is important to consider as it directly affects the low frequency operation of the circuit.
A further consideration for this capacitor is the leakage path from the input source through the input network
(R
S
, C
S
) and the feedback resistor (R
F
) to the load. This leakage current creates a dc-offset voltage at the input
to the amplifier that reduces useful headroom, especially in high gain applications. For this reason, a
low-leakage tantalum or ceramic capacitor is the best choice. When polarized capacitors are used, the positive
side of the capacitor should face the amplifier input in most applications, as the dc level there is held at V
DD
/2,
which is likely higher than the source dc level. It is important to confirm the capacitor polarity in the application.
power supply decoupling, C
PS
The TLV4120 is a high-performance CMOS amplifier that requires adequate power supply decoupling to ensure
stability and low total harmonic distortion (THD). Optimum decoupling is achieved by using two capacitors of
different types that target different types of noise on the power supply leads. For higher frequency transients,
spikes, or digital hash on the line, a good low equivalent-series-resistance (ESR) ceramic capacitor, typically
0.1 µF placed as close as possible to the device V
DD
lead works best. For filtering lower-frequency noise signals,
a larger aluminum electrolytic capacitor of 10 µF or greater placed near the amplifier is recommended.