LMH6553 Datasheet, PDF (19/24 Pages) National Semiconductor (TI) – 900 MHz Fully Differential Amplifier With Output Limiting Clamp

SPLIT SUPPLY OPERATION
For optimum performance, split supply operation is recom-
mended using +5V and −5V supplies; however, operation is
possible on split supplies as low as +2.25V and −2.25V and
as high as +6V and −6V. Provided the total supply voltage
does not exceed the 4.5V to 12V operating specification,
asymmetric supply operation is also possible and in some
cases advantageous. For example, if 5V DC coupled opera-
tion is required for low power dissipation but the amplifier input
common mode range prevents this operation, it is still possi-
ble with split supplies of (V⁺) and (V^−). Where
(V⁺) - (V^−) = 5V and V⁺ and V^− are selected to set the amplifier
input common mode voltage to suit the application.
CLAMP OPERATION
The output clamp allows control of the maximum amplifier
output swing to prevent overdriving of following stages such
as sensitive ADC inputs and provide fast recovery from signal
transients that would otherwise saturate the signal path. Fig-
ure 6 shows the relationship between V_CLAMP and the +OUT
and −OUT outputs. The example circuit shown has a single
ended input and is set for a gain of 2 V/V. For proper operation
V_CM < V_CLAMP < V_CM + 2.0V and the upper single ended output
voltage is limited to the voltage level set at the V_CLAMP input.
The output common mode control loop forces the lower single
ended voltage to be limited to 2*V_CM - V_CLAMP. The maximum
clamped single ended output swing is therefore equal to 2*
(V_CLAMP - V_CM) and the maximum differential output swing is
therefore equal to 4*(V_CLAMP - V_CM). In the example of Figure
6 with V_CLAMP set to 2V and V_CM set to 1.5V, the maximum
single ended output is therefore 1 V_PP centered at 1.5V and
the maximum differential output is 2 V_PP. This is shown for the
case of a 2 V_PP input sine wave which for a gain of 2 V/V in
unclamped operation would provide single ended outputs at
+OUT and -OUT of 2 V_PP but is shown being clamp limited to
1 V_PP.
CLAMP PERFORMANCE
Key clamp performance specifications are listed in the elec-
trical characteristics section. Figure 7 illustrates the clamp
overdrive recovery time which is defined as the difference in
input to output propagation delay due to a step change at the
input for a clamped output versus a normal linear unclamped,
non-saturated output.
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FIGURE 7. Clamp Overdrive Recovery Time
MAXIMUM OUTPUT LEVEL
The maximum unclamped output swing in normal operation
is 4V_PP single ended or 8 V_PP differential due to the require-
ment that V_CLAMP < V_CM + 2.0V. For split supply operation of
+5V and −5V, the maximum output voltage is limited by the
output stage's ability to swing close to either supply
(V_OUT < ±3.7V). As shown in Figure 8, if V_CLAMP is set > 3.7V,
the amplifier output will saturate at the positive supply before
the clamp can operate and similarly if 2*V_CM - V_CLAMP < −3.7V,
the amplifier output will saturate at the negative supply.
FIGURE 6. Clamp Operation
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FIGURE 8. Split Supply V_OUT(MAX) and V_OUT(MIN) Output
Levels
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