LTC1992 datasheet(31/42 Pages) AD

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LTC1992 Family

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APPLICATIONS INFORMATION

Typical Performance plots for small-signal step response,

large-signal step response and gain over frequency to

appraise the effects of capacitive loading. While the con-

sequences are minor in most instances, consider these

effects when designing application circuits with large

capacitive loads.

Input Signal Amplitude Considerations

For application circuits to operate correctly, the amplifier

must be in its linear operating range. To be in the linear

operating range, the input signal’s common mode voltage

must be within the part’s specified limits and the rail-to-rail

outputs must stay within the supply voltage rails. Addition-

ally, the fixed gain LTC1992-X parts have input protection

diodes that limit the input signal to be within the supply

voltage rails. The unconstrained LTC1992 uses external

resistors allowing the source signals to go beyond the

supply voltage rails.

When taken outside of the linear operating range, the

circuit does not perform as expected, however nothing

extreme occurs. Outputs driven into the supply voltage

rails are simply clipped. There is no phase reversal or

oscillation. Once the outputs return to the linear operating

range, there is a small recovery time, then normal opera-

tion proceeds. When the input common mode voltage is

below the specified lower limit, on-chip protection diodes

conduct and clamp the signal. Once the signal returns to

the specified operating range, normal operation proceeds.

If the input common mode voltage goes slightly above the

specified upper limit (by no more than about 500mV),

the amplifier’s open-loop gain reduces and DC offset and

closed-loop gain errors increase. Return the input back to

the specified range and normal performance commences.

If taken well above the upper limit, the amplifier’s input

stage is cut off. The gain servo is now open loop; however,

the common mode servo is still functional. Output bal-

ance is maintained and the outputs go to opposite supply

rails. However, which output goes to which supply rail is

random. Once the input returns to the specified input

common mode range, there is a small recovery time then

normal operation proceeds.

The LTC1992’s input signal common mode range (VINCMR)

is from (–VS – 0.1V) to (+VS – 1.3V). This specification

applies to the voltage at the amplifier’s input, the INP and

INM nodes of Figure 2. The specifications for the fixed gain

LTC1992-X parts reflect a higher maximum limit as this

specification is for the entire gain block and references

the signal at the input resistors. Differential input signals

and single-ended signals require a slightly different set

of formulae. Differential signals separate very nicely into

common mode and differential components while single

ended signals do not. Refer to Figure 5 for the formulae

for calculating the available signal range. Additionally,

Table 1 lists some common configurations and their ap-

propriate signal levels.

The LTC1992’s outputs allow rail-to-rail signal swings. The

output voltage on either output is a function of the input

signal’s amplitude, the gain configured and the output

signal’s common mode level set by the VOCM pin. For

maximum signal swing, the VOCM pin is set at the midpoint

of the supply voltages. For other applications, such as an

ADC driver, the required level must fall within the VOCM

range of (–VS + 0.5V) to (+VS – 1.3V). For single-ended

input signals, it is not always obvious which output will

clip first thus both outputs are calculated and the minimum

value determines the signal limit. Refer to Figure 5 for the

formula and Table 1 for examples.

To ensure proper linear operation both the input common

mode level and the output signal level must be within

the specified limits. These same criteria are also present

with standard op amps. However, with a fully differential

amplifier, it is a bit more complex and old familiar op amp

intuition often leads to the wrong result. This is especially

true for single-ended to differential conversion with level

shifting. The required calculations are a bit tedious, but

are necessary to guarantee proper linear operation.

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