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LT1635

Micropower Rail-to-Rail Op Amp and Reference

Linear Technology 

LT1635

APPLICATIONS INFORMATION

The LT1635 is fully specified with V + = 5V, V – = 0V and

VCM = 2.5V. The op amp offset voltage is internally

trimmed to a minimum value at these supply voltages. A

unique feature of this device is that it operates from a

single 1.2V supply up to ±5V. A full set of specifications is

provided at ±5V supply voltages. The positive supply pin

of the LT1635 should be bypassed with a small capacitor

(about 0.1µF), as well as the negative supply pin when

using split supplies.

Op Amp

The LT1635 is fully specified for single supply operation,

i.e., when the negative supply is 0V. Input common mode

range of the op amp includes ground and the output

swings within a few millivolts of ground while sinking

current. The input stage of the op amp incorporates phase

reversal protection to prevent false outputs from occur-

ring when the input is below the negative supply. Protec-

tive resistors have been included in the input leads so that

current does not become excessive when the inputs are

forced below the negative supply.

The op amp also includes an offset nulling feature, this is

accomplished by connecting the BALANCE pin (Pin 5) to

a variable voltage derived from the reference output. The

offset adjust range is asymmetrical, typically – 2mV to

8mV. At room temperature the input offset voltage of the

LT1635 is within the null range, thus the offset voltage can

be adjusted to zero. Figure 1 shows the standard offset

adjustment.

V+

2–

4

6

LT1635

3+

18

5

VREF

R1

10k

V–

1635 F01

Figure 1. Standard Offset Adjustment

Output

The output voltage swing of the LT1635 is a function of

input overdrive as shown in the typical performance curves.

When monitoring voltages within 15mV of either rail, gain

should be taken to keep the output from saturating. For

example, a 1mV input signal will cause the amplifier to set

up in its linear region in the gain 100 configuration as

shown in Figure 2a. However, 1mV is not enough to make

the amplifier function properly in the voltage follower mode

(Figure 2b).

99R

5V

R–

LT1635

+OP AMP

1mV

5V

–

100mV

1mV

LT1635

+OP AMP

OUTPUT

SATURATED

≈ 2mV

1635 F02

(a)

(b)

Figure 2. Gain 100 Amplifier and Voltage Follower

Distortion

There are two main contributors of distortion in op amps:

distortion caused by nonlinear common mode rejection

and output crossover distortion as the output transitions

from sourcing to sinking current. The common mode

rejection ratio of the LT1635 is very good, typically 110dB.

Therefore, as long as the input operates in normal com-

mon mode range, there will be very little common mode

induced distortion. Crossover distortion will increase as

the output load resistance decreases. For the lowest dis-

tortion, the LT1635 should be operated with the output

always sourcing current.

Reference

The reference of the LT1635 consists of a 200mV precision

bandgap and a reference amplifier. As shown in the block

diagram, the 0.2V precision bandgap is referred to V – and

is internally connected to the noninverting input of the ref-

erence amplifier. This configuration offers great flexibility

in that the reference voltage can be amplified or the reference

amplifier can be used as a comparator. Unlike the op amp,

the output of the reference amplifier can only swing within

0.8V (typ) of the positive rail. To guarantee that the reference

amplifier does not saturate over the industrial temperature

range, the minimum operating supply should be 1.3V. The

reference amplifier can source 2mA of load current and can

sink 10µA over the industrial temperature range.

9

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