LT1677CS8 datasheet(15/20 Pages) LINER

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LT1677

1677fa

TA = 25°C

VS = ±15V

RL CONNECTED TO 0V

MEASURED ON TEKTRONIX 577 CURVE TRACER

TA = 25°C

VS = 5V

RL = 600Ω

MEASURED ON TEKTRONIX 577 CURVE TRACER

Rail-to-Rail Input

The LT1677 has the lowest voltage noise, offset voltage

and highest gain when compared to any rail-to-rail op

amp. The input common mode range for the LT1677 can

exceed the supplies by at least 100mV. As the common

mode voltage approaches the positive rail (+VS – 0.7V),

the tail current for the input pair (Q1, Q2) is reduced,

which prevents the input pair from saturating (refer to the

Simplified Schematic). The voltage drop across the load

resistors RC1, RC2 is reduced to less than 200mV, degrad-

ing the slew rate, bandwidth, voltage noise, offset voltage

and input bias current (the cancellation is shut off).

When the input common mode range goes below 1.5V

above the negative rail, the NPN input pair (Q1, Q2) shuts

off and the PNP input pair (Q8, Q9) turns on. The offset

voltage, input bias current, voltage noise and bandwidth

are also degraded. The graph of Offset Voltage Shift vs

Common Mode shows where the knees occur by display-

ing the change in offset voltage. The change-over points

are temperature dependent, see the graph Common Mode

Range vs Temperature.

APPLICATIO S I FOR ATIO

–5

–10

–15

OUTPUT VOLTAGE (V)

RL = 600

Figure 9. Voltage Gain Split Supply

OUTPUT VOLTAGE (V)

RL TO 5V

Figure 10. Voltage Gain Single Supply

RL TO 0V

Rail-to-Rail Output

The rail-to-rail output swing is achieved by using transis-

tor collectors (Q28, Q29) instead of customary class A-B

emitter followers for the output stage. Referring to the

Simplified Schematic, the output NPN transistor (Q29) sinks

the current necessary to move the output in the negative

direction. The change in Q29’s base emitter voltage is re-

flected directly to the gain node (collectors of Q20 and Q16).

For large sinking currents, the delta VBE of Q29 can domi-

nate the gain. Figure 9 shows the change in input voltage

for a change in output voltage for different load resistors

connected between the supplies. The gain is much higher

for output voltages above ground (Q28 sources current)

since the change in base emitter voltage of Q28 is attenu-

ated by the gain in the PNP portion of the output stage.

Therefore, for positive output swings (output sourcing

current) there is hardly any change in input voltage for any

load resistance. Highest gain and best linearity is achieved

when the output is sourcing current, which is the case in

single supply operation when the load is ground referenced.

Figure 10 shows gains for both sinking and sourcing load

currents for a worst-case load of 600

Ω.

RL = 1k

RL = 10k

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