參數(shù)資料
型號: THS7002CPWP
廠商: TEXAS INSTRUMENTS INC
元件分類: 模擬信號調(diào)理
英文描述: SPECIALTY ANALOG CIRCUIT, PDSO28
封裝: GREEN, PLASTIC, HTSSOP-28
文件頁數(shù): 13/41頁
文件大?。?/td> 1203K
代理商: THS7002CPWP
THS7001, THS7002
70MHz PROGRAMMABLEGAIN AMPLIFIERS
SLOS214C OCTOBER 1998 REVISED MARCH 2007
20
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
APPLICATION INFORMATION
PGA gain control (continued)
One aspect of the THS7001 and THS7002 PGA signal inputs is that there are internal variable resistors (RF
and RG), which set the gain. The resistance of RG changes from about 270- (Gain = +20 dB) to about 3-k
(Gain = 22 dB). Therefore, any source impedance at the input to the PGA amplifiers will cause a gain error
to be seen at the output. A buffer/amplifier is highly recommended to directly drive the input of the PGA section
to help minimize this effect.
Another thing which should be kept in mind is that when each amplifier’s VREF is connected to ground, the
internal RG resistor is connected to a virtual ground. Therefore, if a termination resistor is used on the source
side, the total terminating resistance is the parallel combination of the terminating resistance and the internal
RG resistor. This, in conjunction with the series impedance problem mentioned previously, can potentially cause
a voltage mismatch between the output of a 50-
source and the expected PGA output voltage. These points
can be easily seen in the simplified diagram of the THS7001 and THS7002 PGA section (see Figure 56).
+
PGA
VOUT
No Source Impedance
VIN
RG
RF
PGA
G0 G1 G2
PGA
VIN
Negative
Clamp VL
Positive
Clamp VH
THS7001 and THS7002 IC
PGA
VREF
RTERMINATION
RSOURCE
R
TOTAL TERMINATION +
R
TERMINATION
R
SOURCE )
R
G
R
TERMINATION )
R
SOURCE )
R
G
Figure 56. Simplified PGA Section of the THS7001 and THS7002
voltage reference terminal
If a voltage is applied to the PGA’s VREF terminal, then the output of the PGA section will amplify the applied
reference voltage by one plus the selected gain. Thus, the output gain strictly due to VREF will be from +0.6 dB
to +21 dB according to the following formula:
V
OUT
V
REF
+ 20 X Log
10 1 ) PGA Gain V V
For most configurations, it is recommended that this pin be connected to the signal ground.
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