Datasheet

THS4062 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

and

–R

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. At low frequencies, this

causes the input impedance to ground at the input terminal to look like two

100-Ω resistors in parallel (R1 and R4 for channel 1, and R9 and R12 for

channel 2).

Because the virtual ground is created by the amplifier’s negative feedback

signal, the virtual ground may not hold true at very high frequencies and/or with

very fast input pulses. This will show up as a frequency-dependant input

impedance to the external signal source. To help minimize this, increase the

values of R4 and R12 so that R1 and R9 become dominant in their respective

parallel combination resistances. In simplified form, these equations are:

and

As R4 and R12 increase in value, R1 and R9 must decrease. Because R1 and

R9 are connected to true ground, the frequency dependant input is minimized.

To further assist selecting appropriate resistor values, the above equations

can be reconfigured as shown below.

Channel 1:

–R

and

–R

Channel 2:

–R

and

–R

where R

is the source impedance.