THS7001 Datasheet, PDF (20/32 Pages) Texas Instruments – 70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

THS7001, THS7002
70-MHz PROGRAMMABLE-GAIN AMPLIFIERS
SLOS214B – OCTOBER 1998 – REVISED AUGUST 1999
APPLICATION INFORMATION
PGA gain control (continued)
One aspect of the THS7001 and THS7002 PGA signal inputs is that there are internal variable resistors (R_F
and R_G), which set the gain. The resistance of R_G changes from about 270-Ω (Gain = +20 dB) to about 3-kΩ
(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 V_REF is connected to ground, the
internal R_G 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
R_G resistor. This, in conjunction with the series impedance problem mentioned previously, can potentially cause
a voltage mismatch between the output of a 50-Ω 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).
No Source Impedance
VIN
PGA
–VIN
G0 G1 G2
THS7001 and THS7002 IC
RSOURCE
RG
RTERMINATION
PGA
VREF
RF
–
PGA
+
Positive
Clamp VH
PGA
VOUT
Negative
Clamp VL
+ ₎ ^{ǒǒ )) ǓǓ} RTOTAL _TERMINATION
RTERMINATION
RTERMINATION
RSOURCE RG
RSOURCE RG
Figure 56. Simplified PGA Section of the THS7001 and THS7002
voltage reference terminal
If a voltage is applied to the PGA’s V_REF 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 V_REF will be from +0.6 dB
to +21 dB according to the following formula:
+ ^ƪ ) ^{ǒ ǒ} ń ^{Ǔ Ǔ ƫ VOUT}
VREF
20 X Log₁₀ 1 PGA Gain V V
For most configurations, it is recommended that this pin be connected to the signal ground.
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