THS4521 Datasheet, PDF (28/43 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

THS4521
THS4522
THS4524
SBOS458B – DECEMBER 2008 – REVISED MAY 2009.................................................................................................................................................... www.ti.com
Driving Capacitive Loads
The THS4521, THS4522, and THS4524 family 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. Figure 66 shows the recommended
values of R_O versus capacitive loads (C_L). Figure 67
shows the frequency response with various values.
RECOMMENDED R_O vs C_LOAD
For Flat Frequency Response
1k
100
10 ^{VS+ = 5.0 V}
Gain = 1 V/V
R_F = 1 kW
R_L = 1 kW Differential
V_OUT = 100 mV_PP
1
10
100
C_LOAD (pF)
1000
Figure 66. Recommended Series Output Resistor
versus Capacitive Load for Flat Frequency
Response, with R_LOAD = 1 kΩ
FREQUENCY RESPONSE vs C_LOAD
5
0
R_O = 7.15 W
-5
C_L = 1000 pF each output
R_O = 150 W
C_L = 4.7 pF
each output
-10
R_O = 37.5 W
C_L = 100 pF each output
-15
V_S+ = 5.0 V, Gain = 1 V/V
-20
R_F = 1 kW differential
R_L = 1 kW
R_O = 124 W
C_L = 10 pF
-25 ^{VOUT = 100 mVPP}
each output
0.1
1
10
100
Frequency (MHz)
1000
Figure 67. Frequency Response for Various R_O
and C_L Values, with R_LOAD = 1 kΩ
28
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