Datasheet

THS4082 EVM Inverting Operation

1-8

General

The gain of the EVM can easily be changed to support a particular application

by simply changing the ratio of resistors R6 and R4 (channel 1) and R14 and

R12 (channel 2) as described in the following equation:

Inverting Gain 

–R



–R6

and

–R14

R12

In addition, some applications, such as those for video, may require the use

of 75-Ω cable and 75-Ω EVM input termination and output isolation resistors.

Because the noninverting terminals are at ground potential, the inverting

terminal becomes a

virtual ground

and is held to 0 V. This causes the input

impedance to ground at the input terminal to look like two resistors in parallel

(R1 and R4 for channel 1, and R9 and R12 for channel 2). As a result, if the

source termination is changed, R1 and R9 must be adjusted in accordance

with the following equations:

R1 

R4  R

R4–R

(Channel 1) and R9 

R12  R

R4–R

(Channel 2)

where R

is the source impedance.

Any of the resistors on the EVM board can be replaced with a resistor of a

different value; however, care must be taken because the surface-mount

solder pads on the board are somewhat fragile and will not survive many

desoldering/soldering operations.

Note:

External factors can significantly affect the effective gain of the EVM. For ex-

ample, connecting test equipment with 50-Ω input impedance to the EVM

output will divide the output signal level by a factor of 2 (assuming the output

isolation resistor on the EVM board remains 50 Ω). Similar effects can occur

at the input, depending upon how the input signal sources are configured.

The gain equations given above assume no signal loss in either the input or

the output.

Frequency compensation capacitors C3 and C6 may need to be installed to

improve stability at lower gains. The appropriate value depends on the

particular application.