LTC1992 Family
31
1992fb
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 (V
INCMR
)
is from (–V
S
– 0.1V) to (+V
S
– 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 V
OCM
pin. For
maximum signal swing, the V
OCM
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 V
OCM
range of (–V
S
+ 0.5V) to (+V
S
– 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.
