THS4541_16 Datasheet, PDF (45/61 Pages) Texas Instruments – THS4541 Negative Rail Input, Rail-to-Rail Output, Precision, 850-MHz Fully Differential Amplifier

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THS4541
SLOS375A – AUGUST 2014 – REVISED SEPTEMBER 2014
Typical Applications (continued)
This configuration shows a nominal 18° phase margin (from Table 2); therefore, a very highly-peaked response is
illustrated in Figure 1. This peaking can be eliminated by placing two feedback capacitors across the Rf elements
and a differential input capacitor. Adding these capacitors provides a transition from a resistively set noise gain
(NG1 here; 1.1 in Table 2) to a capacitive divider at high-frequency flattening out to a higher noise gain (NG2
here). The key for this approach is to target a Zo, where the noise gain begins to peak up. Using only the
following terms, and targeting a closed-loop flat (Butterworth) response, gives this solution sequence for Zo and
then the capacitor values.
1. Gain bandwidth product in Hz (850 MHz for the THS4541)
2. Low frequency noise gain, NG1 ( = 1.1 in the attenuator gain of 0.1 V/V design)
3. Target high-frequency noise gain selected to be higher than NG1 (NG2 = 3.1 V/V is selected for this design)
4. Feedback resistor value, Rf (assumed balanced for this differential design = 402 Ω for this design example)
From these elements, for any decompensated voltage-feedback op amp or FDA, solve for Zo (in Hz) using
Equation 15:
Zo
GBP
NG1²
§
¨¨©¹

NG1
NG2

1

2
NG1
NG2
·
¸¸¹
(15)
From this target zero frequency in the noise gain, solve for the feedback capacitors using Equation 16:
Cf 
1
25 W Rf W Zo W NG2
(16)
The next step is to resolve the input capacitance on the summing junction. Equation 17 is for a single-ended op
amp (for example, OPA847) where that capacitor goes to ground. To use Equation 17 for a voltage-feedback
FDA, cut the target value in half, and place the result across the two inputs (reducing the external value by the
specified internal differential capacitance).
Cs NG2  1Cf
(17)
Setting the external compensation elements using Equation 15 to Equation 17 allows an estimate of the resulting
flat bandwidth f_–3dB frequency, as shown in Equation 18:
f_3dB | GBP W Zo
(18)
Running through these steps for the THS4541 in the attenuator circuit of Figure 77 gives the proposed
compensation of Figure 78 where Equation 18 estimates a bandwidth of 252 MHz (Zo target is 74.7 MHz).
C1
1.7 pF
Vcc
+ _Vcc
± 5V
Vcm
+ _Vcm
± 2.5 V
Gain of 0.1 V/V from Rg1,
Single-Ended to Differential,
DC-Coupled, Single Supply
Rg1
4.02 k
+
VG1
±
C3 Vocm
1 pF
Rg2
4.02 k
Rf1
402
Vcc
±
+
FDA
±
+
PD
Vcc
Rf2
402
THS4541 Wideband,
Fully-Differential Amplifier
R1
500
+
VM1
±
C2
1.7 pF
Figure 78. Compensated Attenuator Circuit Using the THS4541
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