THS4521-HT_15 Datasheet, PDF (20/27 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT,RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

THS4521-HT
SBOS548D – APRIL 2011 – REVISED MAY 2012
Low-Power Applications and the Effects of
Resistor Values on Bandwidth
For low-power operation, it may be necessary to
increase the gain setting resistors values to limit
current consumption and not load the source. Using
larger value resistors lowers the bandwidth of the
THS4521 as a result of the interactions between the
resistors, the device parasitic capacitance, and
printed circuit board (PCB) parasitic capacitance.
6.00E+00
1.00E+00
100 kW
10 kW
1 kW
-4.00E+00
-9.00E+00
-1.40E+01
-1.90E+01
-2.40E+01
100000
1000000 10000000 100000000 1000000000
Frequency (Hz)
Figure 38. THS4521 Frequency Response with
Various Gain Setting and Load Resistor Values
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Driving Capacitive Loads
The THS4521 is designed for a nominal capacitive
load of 1 pF on each output to ground. When driving
capacitive loads greater than 1 pF, it is recommended
to use small resistors (R_O) in series with the output,
placed as close to the device as possible. Without
R_O, capacitance on the output interacts with the
output impedance of the amplifier and causes phase
shift in the loop gain of the amplifier that reduces the
phase margin. This reduction in phase margin results
in frequency response peaking; overshoot,
undershoot, and/or ringing when a step or square-
wave signal is applied; and may lead to instability or
oscillation. Inserting R_O isolates the phase shift from
the loop gain path and restores the phase margin, but
it also limits bandwidth.
20
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