DATA SHEET
KH560
R
s
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
the inverting current errors have a gain of simply (R
f
- R
o
)
to the output voltage (neglecting the R
o
to R
L
attenuation).
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.
V
os
R
−
R
o
±
I
bi
(
R
f
−
R
o
)
=
(
I
bn
⋅
R
s
±
V
io
)
⋅
1
+
f
R
g
*
e
ni
√4kTR
s
+
Classical
op-amp
-
√4kT(R
f
- R
o
)
√4kTRV
o
*
*
i
ni
*
*
e
o
R
o
√
where:
4kT
R
g
*
*
i
i
R
g
R
f
- R
o
Gain to e
o
A
v
A
v
R
s
R
f
- R
o
A
v
e
ni
– non-inverting input voltage noise
i
ni
– non-inverting input current noise
i
i
– inverting input current noise
4kTR
s
−
source resis tan ce voltage
noise
4kT / R
g
−
gain settling resistor
noise current
4kT
(
R
f
−
R
o
)
−
feedback resistor
voltage noise
4kTR
o
−
output resistor voltage noise
Figure 8: Equivalent Noise Model
R
f
- R
o
where: I
bn
≡
non
−
inverting bias current
I
bi
≡
inverting bias current
V
io
≡
input offset voltage
An example calculation for the circuit in Figure 1 using
typical 25°C DC error terms and R
s
= 25Ω, R
L
= 50Ω
yields:
V
o
=
(
5
µ
A
⋅
25
Ω ±
2.0mV
)
10
±
10
µ
A
(
360
Ω
)
L
1/ 2
= ±
12.4mV
DC
1
1
[
]
↑
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.
e
o
=
attentuation between R
o
and R
L
Recall that the source impedance, R
s
, includes both the
terminating and signal source impedance and that the
actual DC level to the load includes the voltage divider
between R
o
and R
L
. Also note that for the KH560, as well
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.
(
e
ni
2
+
(
i
ni
R
s
)
+
4kTR
s
A
v 2
+
i
i2
(
R
f
−
R
o
)
L
2
2
)
+
4kT
(
R
f
−
R
o
)
A
v
+
4kTR
o
Where the 4kT(R
f
- R
o
) A
v
term is the combined noise
power of R
g
and R
f
- R
o
.
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. Note that R
L
may be
neglected in this calculation.
e
n
=
e
ni
+
(
i
ni
R
s
)
+
4kTR
s
+
2
2
i
i2
(
R
f
−
R
o
)
A
v2
+
2
+
L
4kT
(
R
f
−
R
o
)
A
v
4kTR
o
A
v2
10
REV. 1A January 2004
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
