參數(shù)資料
型號(hào): THS7001PWP
廠商: Texas Instruments, Inc.
英文描述: 70-MHz PROGRAMMABLE-GAIN AMPLIFIERS
中文描述: 70 - MHz的可編程增益放大器
文件頁數(shù): 21/32頁
文件大小: 597K
代理商: THS7001PWP
THS7001, THS7002
70-MHz PROGRAMMABLE-GAIN AMPLIFIERS
SLOS214B – OCTOBER 1998 – REVISED AUGUST 1999
21
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
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
–VCC
To Bias Circuits
VH
VL
OUT
Output
Transistor
Output
Transistor
Drive
V1
0.1
μ
F
V2
0.1
μ
F
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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