Single/Dual/Quad, Micropower,
Single-Supply Rail-to-Rail Op Amps
the op amp in an inverting configuration (Figure 3a);
connect resistor R3 between the noninverting input and
the input signal when using the op amp in a noninvert-
ing configuration (Figure 3b). Select R3 to equal the
parallel combination of R1 and R2. High source resis-
tances will degrade noise performance, due to the ther-
mal noise of the resistor and the input current noise
(which is multiplied by the source resistance).
(Figure 4). The diodes limit the differential voltage
applied to the amplifiers’ internal circuitry to no more
than V
F
, where V
F
is the diodes’ forward-voltage drop
(about 0.7V at +25°C).
Input bias current for the ICs (±25nA typical) is speci-
fied for the small differential input voltages. For large
differential input voltages (exceeding V
F
), this protec-
tion circuitry increases the input current at IN+ and IN-:
(V
IN
+ - V
IN
- ) - V
F
Input Current = ———————————
2 x 1.7kΩ
For comparator applications requiring large differential
voltages (greater than V
F
), you can limit the input cur-
rent that flows through the diodes with external resistors
MAX492/MAX494/MAX495
Input Stage Protection Circuitry
T
he MAX492/MAX494/MAX495 include internal protec-
tion circuitry that prevents damage to the precision
input stage from large differential input voltages. This
protection circuitry consists of back-to-back diodes
between IN+ and IN- with two 1.7kΩ resistors in series
R2
IN+
R1
V
IN
1.7kΩ
TO INTERNAL
CIRCUITRY
MAX492
MAX494
MAX495
MAX49_
V
OUT
R3
R3 = R2
II
R1
IN–
1.7kΩ
TO INTERNAL
CIRCUITRY
Figure 3a. Reducing Offset Error Due to Bias Current:
Inverting Configuration
Figure 4. Input Stage Protection Circuitry
R3
CAPACITIVE LOAD (pF)
V
IN
UNSTABLE REGION
MAX49_
R2
R3 = R2
II
R1
R1
V
OUT
1000
V
CC
= +5V
V
OUT
= V
CC
/2
R
L
TO V
EE
A
V
= +1
100
1
10
RESISTIVE LOAD (kΩ)
100
Figure 3b. Reducing Offset Error Due to Bias Current:
Figure 5. Capacitive-Load Stable Region Sourcing Current
Noninverting Configuration
______________________________________________________________________________________ 11
MAX492-FG 04
10,000
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
