Rainbow Electronics MAX495 - Manual (Page 10)

MAX492/MAX494/MAX495
Single/Dual/Quad, Micropower,
Single-Supply Rail-to-Rail Op Amps
10 ______________________________________________________________________________________
MAX495
10k
1
5
NULL
V
EE
4
NULL
Figure 2. Offset Null Circuit
__________Applications Information
The dual MAX492, quad MAX494, and single MAX495
op amps combine excellent DC accuracy with rail-to-
rail operation at both input and output. With their preci-
sion performance, wide dynamic range at low supply
voltages, and very low supply current, these op amps
are ideal for battery-operated equipment and other low-
voltage applications.
Rail-to-Rail Inputs and Outputs
The MAX492/MAX494/MAX495’s input common-mode
range extends 0.25V
beyond the positive and negative
supply rails, with excellent common-mode rejection.
Beyond the specified common-mode range, the out-
puts are guaranteed not to undergo phase reversal or
latchup. Therefore, the MAX492/MAX494/MAX495 can
be used in applications with common-mode signals at
or even beyond the supplies, without the problems
associated with typical op amps.
The MAX492/MAX494/MAX495’s output voltage swings
to within 50mV of the supplies with a 100k
Ω
load. This
rail-to-rail swing at the input and output substantially
increases the dynamic range, especially in low supply-
voltage applications. Figure 1 shows the input and out-
put waveforms for the MAX492, configured as a
unity-gain noninverting buffer operating from a single
+3V supply. The input signal is 3.0V
p-p , 1kHz sinusoid
centered at +1.5V. The output amplitude is approxi-
mately 2.95V
p-p .
Input Offset Voltage
Rail-to-rail common-mode swing at the input is obtained
by two complementary input stages in parallel, which
feed a folded cascaded stage. The PNP stage is active
for input voltages close to the negative rail, and the
NPN stage is active for input voltages close to the posi-
tive rail.
The offsets of the two pairs are trimmed; however, there
is some small residual mismatch between them. This
mismatch results in a two-level input offset characteris-
tic, with a transition region between the levels occurring
at a common-mode voltage of approximately 1.3V.
Unlike other rail-to-rail op amps, the transition region
has been widened to approximately 600mV in order to
minimize the slight degradation in CMRR caused by
this mismatch.
To adjust the MAX495’s input offset voltage (500µV max
at +25°C), connect a 10k
Ω
trim potentiometer between
the two NULL pins (pins 1 and 5), with the wiper con-
nected to V
EE (pin 4) (Figure 2). The trim range of this
circuit is ±6mV. External offset adjustment is not avail-
able for the dual MAX492 or quad MAX494.
The input bias currents of the MAX492/MAX494/MAX495
are typically less than 50nA. The bias current flows into
the device when the NPN input stage is active, and it
flows out when the PNP input stage is active. To reduce
the offset error caused by input bias current flowing
through external source resistances, match the effec-
tive resistance seen at each input. Connect resistor R3
between the noninverting input and ground when using
V IN
V OUT
Figure 1. Rail-to-Rail Input and Output (Voltage Follower
Circuit, VCC = +3V, VEE = 0V)