OP-160_15 Datasheet, PDF (21/24 Pages) Analog Devices – High-Speed, Current Feedback Operational Amplifier

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Page 22
OP-160
At higher frequencies the gain increases or peaks due to the
effect of the parasitic capacitance, Cs. on the gain equation. Any
capacitance at the inverting input will create a zero In the ampli-
fier's response. This fact can be used to compensate for gain
roll-off due to loading on the amplifier,
Begin by measuring or estimating the amplifier's -6dB point
(this is the freq uency at which the output signal is half its original
amplitude). This can be easily determined from a network
will be increased. At higher gains, gain flatness can be signifi-
cantly improved without gain peaking. Figure 28 depicts the
OP-160 with Av = + 10. In this example f--6dB'" 22 MHz so.
1
Cs = 9pF +
27t(91 Q)22MHz
1
+
21t(820Q)22MHz
= 97pF
The nearest standard capacitor value is 100pF.
analyzer plot of the amplifier's frequency performance. From
this the amount of capacitance, Cc' which will double the gain
at the -6dB frequency and restore the original gain, can be
determined.
Gain periormance is flat to O.5dB to 30MHz and the amplifier's
-3dB point is 38MHz. This gives the amplifier an effective gain-
bandwidth of 380MHz! Compensating the OP-160 does not ef-
fect the pulse response as shown in Figure 29.
From the -6dB frequency, Cc can be calculated:
1
1
Cc= Cs+
+-
2nR1f-6dB
27tR2f-6dB
for non inverting configuration, where Cs is the combination of
l _{the amplifier's input capacitance and the stray capacitance at}
the input.
O_{In the example shown,}
B Cs = 9pF = OP-160 input capacitance (4pF) + stray capacitance
(5pF)
S ₁
Cs = 9pF +.
O 2n(910n)32MHz
1
+
-
27t(910n)32MHz
= 20pF
L _{Figure 27 is an expanded scale plot of the gain performance of}
E _{the compensated amplifier at Av = +2. Gain performance is flat}
TE ^{to :to. 1dB out to beyond 9MHz. For low gains (Av S 5) peaking}
26
24
22
20
ro 18
:8.
z 16
~
<.!} 14
12
10
8
6
1
10
100
FREQUENCY (MHz)
FIGURE 28; Gain/frequency graph for the compensated ampli-
fier. Av .. + 10, showing the effect of the compensation capaci-
[I"'-
8.3
tance, Ce, on gain flatness.
- ^{TII +25°C}
6.2
Ay +2
I
Cc = 20pF
= ^8.1 r-
RI- RF = 9100
AL 500i1
CD
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Vs..i15V
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<.!} 5.9
5.8
I
j
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INPUT
OUTPUT
5.7
5.6
1
!
;
,ⁱ
!
2
3 4 5 6 7 8 910
i
i
FREQUENCY (MHz)
L
FIGURE 29; Pulse Response of the OP-160 in a Gain of + 10
Compensated for Gain Flatness
FIGURE 27: Expanded Gain/Frequency Graph of the Compen-
sated Amplifier, Av = +2
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