MAX477 Maxim, MAX477 Datasheet - Page 9

• Manufacturer Part Number: MAX477
• Description: 300MHz High-Speed Op Amp
• Manufacturer: Maxim

Then:
3) The MAX477’s output-referred noise is simply total
In the above example, with e
ing a signal bandwidth of 300MHz (471MHz noise
bandwidth), total output noise in this bandwidth is:
dominated by offset voltage (V
age (e
offset and noise errors significantly higher, the calcula-
tions are very different.
The MAX477 provides maximum AC performance with
no output load capacitance. This is the case when the
MAX477 is driving a correctly terminated transmission
line (i.e., a back-terminated 75Ω cable). However, the
MAX477 is capable of driving capacitive loads up to
100pF without oscillations, but with reduced AC perfor-
mance.
Driving large capacitive loads increases the chance of
oscillations in most amplifier circuits. This is especially
true for circuits with high loop gain, such as voltage fol-
lowers. The amplifier’s output resistance and the load
capacitor combine to add a pole and excess phase to
the loop response. If the frequency of this pole is low
enough and phase margin is degraded sufficiently,
oscillations may occur.
A second problem when driving capacitive loads
results from the amplifier’s output impedance, which
looks inductive at high frequency. This inductance
forms an L-C resonant circuit with the capacitive load,
which causes peaking in the frequency response and
degrades the amplifier’s gain margin.
As an example, consider R
Note that for both DC and noise calculations, errors are
R
e
e
R
e
T
EQ
input-referred noise, e
factor:
OUT
n
75
). For a current-mode feedback amplifier with
= 5.5nV x
4
5
KT x
nV
e
2
500
OUT
325
_______________________________________________________________________________________
2 3
.
1
= e
||
nV
500
Driving Capacitive Loads
500
500
2 3
T
2
.
S
nV
1+
T
= 75Ω, R f = R
T
2
, multiplied by the gain
/
x
pA x
R
R
= 5.5nV √ Hz, and assum-
OS
325
g
Hz at
f
) and input noise volt-
471
325
MHz
25
2
g
C
5 5
239
= 500Ω.
.
nV Hz
300MHz High-Speed Op Amp
V
RMS
The MAX477 drives capacitive loads up to 100pF with-
out oscillation. However, some peaking (in the frequen-
cy domain) or ringing (in the time domain) may occur.
This is shown in Figure 4 and the in the Small and
Large-Signal Pulse Response graphs in the Typical
Operating Characteristics .
To drive larger-capacitance loads or to reduce ringing,
add an isolation resistor between the amplifier’s output
and the load, as shown in Figure 5.
The value of R
capacitive load. Figure 6 shows the Bode plots that
result when a 20Ω isolation resistor is used with a volt-
age follower driving a range of capacitive loads. At the
higher capacitor values, the bandwidth is dominated by
the RC network, formed by R
of the amplifier itself is much higher. Note that adding
an isolation resistor degrades gain accuracy. The load
and isolation resistor form a divider that decreases the
voltage delivered to the load.
Figure 3. Output Offset Voltage
Figure 4. Effect of C
(A
VCL
= +1V/V)
V
IN
-10
-15
-20
15
10
-5
5
0
1M
ISO
R
R
S
g
LOAD
depends on the circuit’s gain and the
on Frequency Response
10M
FREQUENCY (Hz)
I
I
B+
B-
C
C
L
C
L
= 100pF
L
= 41pF
ISO
MAX477
= 0pF
R
and C
f
100M
C
L
L
; the bandwidth
= 22pF
1G
V
OUT
9