THS7002 datasheet(20/32 Pages) TI

20 / 32 page

THS7001, THS7002

70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

SLOS214B – OCTOBER 1998 – REVISED AUGUST 1999

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-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 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-

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

VOUT

No Source Impedance

VIN

PGA

G0 G1 G2

PGA

–VIN

Negative

Clamp VL

Positive

Clamp VH

THS7001 and THS7002 IC

PGA

VREF

RTERMINATION

RSOURCE

TOTAL TERMINATION +

TERMINATION

SOURCE )

TERMINATION )

SOURCE )

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:

OUT

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.

THS7002 Datasheet(PDF) 20 Page - Texas Instruments

THS7002 Datasheet(HTML) 20 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL