DATA SHEET

KH560

10

REV. 1A January 2004

and the impact internal amplifier characteristics have on

the signal gain.

Both the output DC error and noise

model may be developed using the equivalent model of

Figure 5. Generally, non-inverting input errors show up

at the output with the same gain as the input signal, while

Output DC Offset:

The DC error terms shown in the specification listing

along with the model of Figure 5 may be used to estimate

the output DC offset voltage and drift. Each term shown

in the specification listing can be of either polarity. While

the equations shown below are for output offset voltage,

the same equation may be used for the drift with each

term replaced by its temperature drift value shown in the

specification listing.

terminating and signal source impedance and that the

actual DC level to the load includes the voltage divider

as for all current feedback amplifiers, the non-inverting

and inverting bias currents do not track each other in

either magnitude or polarity. Hence, there is no meaning

in an offset current specification, and source impedance

matching to cancel bias currents is ineffective.

Noise Analysis:

Although the DC error terms are in fact random, the cal-

culation shown above assumes they are all additive in a

worst case sense. The effect of all the various noise

sources are combined as a root sum of squared terms to

get an overall expression for the spot noise voltage. The

circuit of Figure 8 shows the equivalent circuit with all the

various noise voltages and currents included along with

their gains to the output.

where:

1

1

Figure 8: Equivalent Noise Model

To get an expression for the equivalent output noise volt-

age, each of these noise voltage and current terms must

be taken to the output through their appropriate gains

and combined as the root sum of squares.

It is often more useful to show the noise as an equivalent

input spot noise voltage where every term shown above

is reflected to the input. This allows a direct measure of

the input signal to noise ratio. This is done by dividing

every term inside the radical by the signal voltage gain

squared. This, and an example calculation for the circuit

of Figure 1, are shown below.

neglected in this calculation.

VI

R

V

1

RR

R

IR

R

where: I

non inverting bias current

I

inverting bias current

V

input offset voltage

V

5 A 25

2.0mV 10 10 A 360

12.4mV

attentuation between R and R

os

bn

s

io

f

o

g

bi

f

o

bn

bi

io

o

oL

1/ 2

=⋅

±

−











 ±−

()

≡−

≡

≡

=⋅

±

()

[]

=±

↑

µµ

Ω

Ω L

An example calculation for the circuit in Figure 1 using

yields:

Classical

op-amp

+

-

*

*

*

*

*

*

*

4

4

4

4

kTR

source resis

ce voltage

noise

kT R

gain settling resistor

noise current

kT R

R

feedback resistor

voltage noise

kTR

output resistor voltage noise

s

g

f

o

o

−

−

−

−

tan

/

e

e

i R

kTR

A

i

R

R

kT R

R

A

kTR

o

ni

ni s

s

v

i

f

o

f

ov

o

=+

()

+−

()

+

2

2

22

2

4

44

L

e

e

i R

kTR

iR

R

A

kT R

R

A

kTR

A

n

ni

ni s

s

i

f

o

v

f

o

v

o

v

=+

−

()

+

−

()

+

2

2

2

2

2

2

4

4

4

L

DC