S in g le /Du a l/Qu a d , Mic ro p o w e r,
S in g le -S u p p ly Ra il-t o -Ra il Op Am p s
2/MAX495
the op amp in an inverting configuration (Figure 3a);
(Fig ure 4). The d iod e s limit the d iffe re ntia l volta g e
applied to the amplifiers internal circuitry to no more
than V , where V is the diodes forward-voltage drop
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).
’
’
F
F
(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 ), this protec-
F
tion circuitry increases the input current at IN+ and IN-:
In p u t S t a g e P ro t e c t io n Circ u it ry
The 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
(V + - V - ) - V
F
IN
IN
Input Current = ———————————
2 x 1.7kΩ
For comparator applications requiring large differential
voltages (greater than V ), you can limit the input cur-
F
rent that flows through the diodes with external resistors
MAX492
MAX494
MAX495
R2
TO INTERNAL
CIRCUITRY
1.7kΩ
IN+
IN–
R1
V
IN
V
OUT
MAX49_
R3
R3 = R2 R1
II
TO INTERNAL
CIRCUITRY
1.7kΩ
Figure 3a. Reducing Offset Error Due to Bias Current:
Inverting Configuration
Figure 4. Input Stage Protection Circuitry
10,000
R3
V
IN
UNSTABLE REGION
V
OUT
MAX49_
R2
R1
1000
R3 = R2 R1
II
V
V
OUT
= +5V
= V /2
CC
CC
R TO V
L
EE
A = +1
V
100
1
10
RESISTIVE LOAD (kΩ)
100
Figure 3b. Reducing Offset Error Due to Bias Current:
Noninverting Configuration
Figure 5. Capacitive-Load Stable Region Sourcing Current
______________________________________________________________________________________ 11
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
