OP-160_15 Datasheet, PDF (14/24 Pages) Analog Devices – High-Speed, Current Feedback Operational Amplifier

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Page 15
OP-160
r ^a)
I
is further reduced and the amplifier remains stable. Figure 7
shows theOP-160 in a gain of + 1 and -1 driving a 1000pF load
without any sign of oscillation. Table 1 shows the effects of
capacitive load on the -3dB bandwidth for Av = -1.
,,= +1
R, =8200
RL = 5OOU
= CL 1000pF
VS.:t15V
b)
TABLE1: -3dB Bandwidth vs. Capacitive Load; Av" -1,
RF.. 820n, RL.. soon, Vs =d:15V.
CAPACITANCE (pF)
0
-3dB BANDWIDTH (MHz)
55
20
55
50
50
75
48
"=-1
Rf = 8200
Rl = 50011
= CL 1000pF
Vs =:t15V
O_{FIGURE 7: The OP-160 remains stable when driving large}
B ^{capacitive loads.}
S _{DRIVING CAPACITIVE LOADS}
O ^{The OP-160 is capable of driving capacitive loads athigh speed.}
Output stage compensation is used to reduce the effects of
L ^{capacitive loading. With low capacitive loads, the gain from the}
compensation node to the output is unity and Co does not con-
E _{tribute to the overall compensation. As the load capacitance is}
T ^{increased, a pole is formed with the output resistance of the}
E ^{amplifier. The gain is reduced and Co begins to contribute to the}
100
40
200
24
500
13
1000
9
AMPLIFIER NOISE PERFORMANCE
Simplified noise models of the OP-160 in the noninverting and
inverting amplifier configurations are shown in Figure 8. All re-
sistors are assumed to be noiseless.
For the noninverting amplifier, the equivalent input voltage
noise, referred to the input, is:
EN ..~(Rs innf + en2 +(R2 inif/AvcL
where:
EN == total input referred noise
en ==amplifier voltage noise
inn ==noninverting input current noise
overall compensation capacitance leading to a reduction in
~I = inverting input current noise
bandwidth. As the load capacitance is increased, the bandwidth
lis .. source resistance
AVCL==closed loop gain ==1 + R:lR1
As
a)
"'IVIN
"::'
"::'
R2
2.5kO
=
Your
b)
R2
R1
",IVIN
"::"
"::"
-=-
vour
FIGURE 8: Simplified noise models for the OP-160 in noninv9rting (a) and inverting (b) gain.
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