THS7001_16 Datasheet, PDF (21/41 Pages) Texas Instruments – 70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

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THS7001, THS7002
70ĆMHz PROGRAMMABLEĆGAIN AMPLIFIERS
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SLOS214C − OCTOBER 1998 − REVISED MARCH 2007
APPLICATION INFORMATION
output clamping
Typically, the output of the PGA will directly drive an analog-to-digital converter (ADC). Because of the limited
linear input range and saturation characteristics of most ADCs, the PGA’s outputs incorporate a voltage clamp.
Unlike a lot of clamping amplifiers which clamp only at the input, the THS7001 and THS7002 clamps at the
output stage. This insures that the output will always be protected regardless of the Gain setting and the input
voltage. The clamps activate almost instantaneously and recover from saturation in less than 7 ns. This can be
extremely important when the THS7001 and THS7002 is used to drive some ADCs which have a very long
overdrive recovery time. It is also recommended to add a pair of high frequency bypass capacitors to the clamp
inputs. These capacitors will help eliminate any ringing which may ocur when a large pulse is applied to the
amplifier. This pulse will force the clamp diodes to abruptly turn on, drawing current from the reference voltages.
Just like a power supply trace, you must minimize the inductance seen by the clamp pins. The bypass capacitors
will supply the sudden current demands when the clamps are suddenly turned on. A simplified clamping circuit
diagram is shown in Figure 57.
+VCC
Drive
To Bias Circuits
VH
Output
Transistor
OUT
V1
0.1 µF
Output
Transistor
VL
V2
0.1 µF
−VCC
Figure 57. Simplified THS7001 and THS7002 Clamp Circuit
Because the internal clamps utilize the same clamping reference voltages, the outputs of both PGAs on the
THS7002 are clamped to the same values. These clamps are typically connected to the power supply pins to
allow a full output range. But, they can be connected to any voltage reference desired. The clamping range is
limited to +V_CC and GND for V_H and –V_CC and GND for V_L. It is possible to go beyond GND for each respective
clamp, but it is not recommended. This is because this operation relies on the internal bias currents in the Class
AB output stage to maintain their linearity. There may also be a chance to reverse bias the PN junctions and
possibly cause internal damage to these junctions. But for reference, the graphs in Figure 58 show the output
voltage versus the clamping voltage with different loads.
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