THS4531A_16 Datasheet, PDF (32/59 Pages) Texas Instruments – Ultra Low-Power, Rail-to-Rail Output, Fully Differential Amplifier

THS4531A
SLOS823C – DECEMBER 2012 – REVISED JANUARY 2016
www.ti.com
9
6
3
0
−3
−6
−9
−12
−15
−18
−21
100k
V_OUT = 100 mV_PP
R_F = 2 kΩ
R_F = 10 kΩ
R_F = 100 kΩ
1M
10M
Frequency (Hz)
100M
G067
Figure 76. THS4531A Frequency Response with Various Gain Setting Resistor Values
9.1.9 Driving Capacitive Loads
The THS4531A is designed for a nominal parasitic capacitive load of 2 pF (differentially). When driving capacitive
loads greater than this, TI recommends using small resisters (R_O) in series with the output as close to the device
as possible. Without R_O, capacitance on the output interacts with the output impedance of the amplifier causing
phase shift in the loop gain of the amplifier that reduces the phase margin resulting in:
• Peaking in the frequency response.
• Overshoot, undershoot, and ringing in the time domain response with a pulse or square-wave signal.
• May lead to instability or oscillation.
Inserting R_O compensates the phase shift and restores the phase margin, but it also limits bandwidth. The circuit
shown in Figure 71 is used to test for best R_O versus capacitive loads, C_L, with a capacitance placed differential
across the V_OUT+ and V_OUT- along with 2-kΩ load resistor, and the output is measure with a differential probe.
Figure 77 shows the suggested values of R_O versus capacitive loads, C_L, and Figure 78 shows the frequency
response with various values. Performance is the same on both 2.7-V and 5-V supply.
200
100
10
V_S = 2.7 V
G = 1 V/V
R_F = 2 kΩ
R_L = 2 kΩ
1
1
10
100
1k 2k
C_LOAD (pF)
G068
Figure 77. Recommended Series Output Resistor vs
Capacitive Load for Flat Frequency Response
3
0
V_S = 2.7 V
−3
G = 1 V/V
−6
R_F = 2 kΩ
R_L = 2 kΩ
−9
V_OUT = 100 mV_pp
−12
−15
−18
−21
100k
C_L = 0 pF, R_O = 0 Ω
C_L = 15 pF, R_O = 200 Ω
C_L = 39 pF, R_O = 100 Ω
C_L = 120 pF, R_O = 50 Ω
C_L = 470 pF, R_O = 20 Ω
C_L = 1200 pF, R_O = 12 Ω
1M
10M
100M
Frequency (Hz)
G069
Figure 78. Frequency Response for Various R_O and C_L
Values
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