10

LT1223

S

APPLICATI

I FOR ATIO

inverting input (A1) senses the shield and the non-invert-

ing input (A2) senses the center conductor. Since this

amplifier does not load the cable (take care to minimize

stray capacitance) and it rejects common mode hum and

noise, several amplifiers can sense the signal with only

one termination at the end of the cable. The design

equations are simple. Just select the gain you need (it

should be two or more) and the value of the feedback

The bandwidth of the noninverting input signal is not

reduced by the presence of the other amplifier, however,

the inverting input signal bandwidth is reduced since it

passes two amplifiers. The CMRR is good at high frequen-

cies because the bandwidth of the amplifiers are about the

same even though they do not necessarily operate at the

same gain.

Summing Amplifier (DC Accurate)

The summing amplifier is easily made by adding additional

inputs to the basic inverting amplifier configuration. The

between the two input bias currents. Therefore, we will not

improve the DC accuracy of the inverting amplifier by

putting in the extra resistor in the noninverting input.

V

= –R

LT1223 • TA07

+

–

In

V

OUT

V

LT1223

+

I1

V

I2

V

1

R

2

R

n

R

V

V

V

I1

I2

In

G1

G2

Gn

(

)

OUT

F

•

•

•

R

R

R

+

G

G

G

R

F

Difference Amplifier

The LT1223 difference amplifier delivers excellent

performance if the source impedance is very low. This is

because the common mode input resistance is only equal

1k

LT1223 • TA09

OUT

V

A1

LT1223

IN

V

A2

LT1223

IN

V

–+

++

––

1k

1k

1k

IN

G – 1

LT1223 • TA08

+

–

OUT

V

LT1223

100

OPTIONAL TRIM

FOR CMRR

(V – V )

1

2

Video Instrumentation Amplifier

This instrumentation amplifier uses two LT1223s to in-

crease the input resistance to well over 1M. This makes an

excellent “loop through” or cable sensing amplifier if the

Cable Driver

The cable driver circuit is shown on the front page. When

driving a cable it is important to properly terminate both

ends if even modest high frequency performance is

required. The additional advantage of this is that it isolates

the capacitive load of the cable from the amplifier so it can

operate at maximum bandwidth.