12

LT1498/LT1499

Figure 1. LT1498 Simplified Schematic Diagram

Input Offset Voltage

The offset voltage changes depending upon which input

stage is active. The input offsets are random, but are

trimmed to less than 475

µV. To maintain the precision

entire input common mode range (CMRR) is guaranteed

to be less than 425

µV on a single 5V supply.

Input Bias Current

The input bias current polarity also depends on the input

common mode voltage, as described in the previous

section. When the PNP differential pair is active, the input

bias currents flow out of the input pins; they flow in

opposite direction when the NPN input stage is active. The

offset error due to input bias current can be minimized by

equalizing the noninverting and inverting input source

impedances. This will reduce the error since the input

offset currents are much less than the input bias currents.

Overdrive Protection

To prevent the output from reversing polarity when the

input voltage exceeds the power supplies, two pair of

crossing diodes D1 to D4 are employed. When the input

Q4

Q6

D6

D5

+IN

D2

Q3

Q7

Q1

Q9

Q2

D4

D1

D3

–IN

OUT

Q5

Q12

Q10

Q8

Q14

1498/99 F01

C1

R1

R6

R3

R4

R5

C2

R2

Q11

Q13

Q15

BUFFER

AND

OUTPUT BIAS

R7

voltage exceeds either power supply by approximately

700mV, D1/D2 or D3/D4 will turn on, forcing the output to

the proper polarity. For the phase reversal protection to

work properly, the input current must be less than 5mA. If

the amplifier is to be severely overdriven, an external

resistor should be used to limit the overdrive current.

Furthermore, the LT1498/LT1499’s input stages are pro-

tected by a pair of back-to-back diodes, D5/D6. When a

differential voltage of more than 0.7V is applied to the

inputs, these diodes will turn on, preventing the Zener

breakdown of the input transistors. The current in D5/D6

should be limited to less than 10mA. Internal resistors R6

and R7 (700

Ω total) limit the input current for differential

input signals of 7V or less. For larger input levels, a

resistor in series with either or both inputs should be

used to limit the current. Worst-case differential input

voltage usually occurs when the output is shorted to

ground. In addition, the amplifier is protected against

ESD strikes up to 3kV on all pins.

Capacitive Load

The LT1498/LT1499 are designed for ease of use. The

amplifier can drive a capacitive load of more than 10nF