THS4551 Datasheet, PDF (44/70 Pages) Texas Instruments – Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

THS4551
SBOS778A – APRIL 2016 – REVISED AUGUST 2016
www.ti.com
From this target zero frequency in the noise gain, the feedback capacitors can be solved as Equation 12:
C_F
1
2S ‡ R_F ‡ Z_O ‡ NG2
(12)
The next step is to resolve the input capacitance on the summing junction. Equation 13 is for a single-ended op
amp where the capacitor goes to ground. To use the capacitance (C_S) resulting from Equation 13 for a voltage-
feedback FDA, cut the target value in half and place the resulting C_S across the two inputs (reducing the external
value by the specified internal differential capacitance).
C_S NG2 1 C_F
(13)
Using the computed capacitor values allows for an estimate of the resulting flat response bandwidth f_–3dB
frequency, as shown in Equation 14:
f _3dB | GBP ‡ Z_O
(14)
Running through these steps for the THS4551 in the attenuator circuit of Figure 80 provides the proposed
compensation of Figure 81, where Equation 14 estimates a bandwidth of 22 MHz (the Z_O target is 3.5 MHz). The
solutions for C_F gives 9 pF, where this value is reduced to 8.4 pF to account for the internal 0.6-pF feedback.
The single-ended solution for C_S gives 36 pF, which is reduced to 18 pF to be differential, and is then further
reduced to 16.8 pF to account for the internal 1.2-pF differential input capacitance of the THS4551.
Gain of 0.1 V/V,
DC-Coupled,
Single-Ended Source to
Differential Output
CF1
8.4 pF
RF1
1k
THS4551 Wideband,
Fully Differential Amplifier
VS+
VS-
+
2.5 V
-2.5 V ⁺
Vin
±
±
RG1
10 k
CS
16.8 pF
VOCM
VS+
±
+
FDA
±
+
PD
VS- ^VS+
VOUT
RL
1k
RG2
10 k
RF2
1k
CF2
8.4 pF
Copyright © 2016, Texas Instruments Incorporated
Figure 81. Compensated Attenuator Circuit Using the THS4551
The 16.8 pF across the inputs is really a total of 36 pF for a single-ended design from Equation 13 reduced by
half and then the 1.2-pF internal capacitance is removed.
These two designs (with and without the compensation capacitors) were both bench tested and simulated using
the THS4551 TINA-TI™ model, which resulted in Figure 82.
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