MAX477 View Datasheet(PDF) - Maxim Integrated
Part Name
Description
Manufacturer
MAX477 Datasheet PDF : 12 Pages
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300MHz High-Speed Op Amp
RG
RF
VOUT
MAX477
VIN
VOUT = [1 + (RF/RG)] VIN
Figure 1b. Noninverting Gain Configuration
equation, while Figure 1b shows the noninverting gain
configuration.
Choosing Resistor Values
The feedback and input resistor values are not critical
in the inverting or noninverting gain configurations (as
with current-feedback amplifiers). However, be sure to
select resistors that are small and noninductive.
Surface-mount resistors are best for high-frequency cir-
cuits. Their material is similar to that of metal-film resis-
tors, but to minimize inductance, it is deposited in a flat,
linear manner using a thick film. Their small size and
lack of leads also minimize parasitic inductance and
capacitance.
The MAX477’s input capacitance is approximately 1pF.
In either the inverting or noninverting configuration,
excess phase resulting from the pole frequency formed
by Rf || Rg and C can degrade amplifier phase margin
and cause oscillations (Figure 2). Table 1 shows the
recommended resistor combinations and measured
bandwidth for several gain values.
DC and Noise Errors
The standard voltage-feedback topology of the
MAX477 allows DC error and noise calculations to be
done in the usual way. The following analysis shows
RG
RF
VIN
C
VOUT
MAX477
RL
Table 1. Resistor and Bandwidth Values for
Various Closed-Loop Gain Configurations
GAIN
Rg
(V/V)
(Ω)
Rf
(Ω)
-3dB
BANDWIDTH
(MHz)
+1
Open
Short
300
+2
500
500
120
+5
125
500
25
+10
50
450
12
-1
300
300
114
-2
150
300
64
-5
100
500
42
-10
50
500
23
that the MAX477’s voltage-feedback architecture pro-
vides a precision amplifier with significantly lower DC
errors and lower noise compared to CMF amplifiers.
1) In Figure 3, total output offset error is given by:
VOUT
=
1+
Rf
Rg
VOS
+
IBRS
–
IB Rf
||Rg
+
IOS RS
+
Rf
||Rg
For the special case in which RS is arranged to be
equal to Rf || Rg, the IB terms cancel out. Note also,
for IOS (RS + (Rf || Rg) << VOS, the IOS term also
drops out of the equation for total DC error. In prac-
tice, high-speed configurations for the MAX477
necessitate the use of low-value resistors for RS, Rf,
and Rg. In this case, the VOS term is the dominant
DC error source.
2) The MAX477’s total input-referred noise in a closed-
loop feedback configuration can be calculated by:
( ) eT =
en2 + eR2 +
inREQ
2
where en = input-referred noise voltage of the
MAX477 (5nV√Hz)
in = input-referred noise current of the
MAX477 (2pA√Hz)
REQ = total equivalent source resistance at
the two inputs, i.e., REQ = RS + Rf || Rg
eR = resistor noise voltage due to REQ, i.e.,
Figure 2. Effect of High-Feedback Resistor Values and
Parasitic Capacitance on Bandwidth
eR = 4KT REQ
8 _______________________________________________________________________________________
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