Datasheet
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 (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).
+
−
PGA
V
OUT
No Source Impedance
V
IN
R
G
R
F
PGA
G
0
G
1
G
2
PGA
−V
IN
Negative
Clamp V
L
Positive
Clamp V
H
THS7001 and THS7002 IC
PGA
V
REF
R
TERMINATION
R
SOURCE
R
TOTAL TERMINATION
+
R
TERMINATION
ǒ
R
SOURCE
) R
G
Ǔ
R
TERMINATION
)
ǒ
R
SOURCE
) R
G
Ǔ
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:
V
OUT
V
REF
+ 20 X Log
10
ƪ
1 )
ǒ
PGA Gain
ǒ
VńV
Ǔ
Ǔ
ƫ
For most configurations, it is recommended that this pin be connected to the signal ground.