DATA SHEET
KH561
at the output with the same gain as the input signal, while
the inverting current errors have a gain of simply (R - R )
Rs
+
*
eni
f
o
√4kTRVo
Classical
op-amp
ini
√4kTRs
to the output voltage (neglecting the R to R attenuation).
eo
o
L
*
*
*
Ro
-
Output DC Offset:
√4kT(Rf - Ro)
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.
*
Rf - Ro
4kT
Rg
Rg
i
√
* * i
where:
Gain to e
o
e – non-inverting input voltage noise
i – non-inverting input current noise
A
v
ni
A R
ni
v s
i – inverting input current noise
R - R
i
f
o
A
R −R
4kTR − source resistance voltage
v
f
o
s
V
= I R ± V
1+
± I R −R
f
bi o
(
)
(
)
os
bn
s
io
noise
R
g
where: I ≡ non − inverting bias current
R - R
bn
f
o
4kT /R − gain settling resistor
g
I
≡ inverting bias current
bi
noise current
V ≡ input offset voltage
io
1
1
4kT R −R − feedback resistor
(
)
f
o
An example calculation for the circuit in Figure 1 using
typical 25°C DC error terms and R = 25Ω, R = 50Ω
yields:
voltage noise
s
L
4kTR − output resistor voltage noise
o
V
= 5µA 25Ω ± 2.0mV 10 ±10µA 360Ω L
(
[
)
(
)
]
o
Figure 8: Equivalent Noise Model
1/ 2 = ±12.4mV
DC
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.
↑
attentuationbetweenR andR
o
L
Recall that the source impedance, R , includes both the
s
terminating and signal source impedance and that the
actual DC level to the load includes the voltage divider
2
2
2
2
2
e =
e
+ i R
+ 4kTR
A
+ i R −R
L
(
)
(
)
o
ni
ni
s
s
v
i
f
o
(
)
between R and R . Also note that for the KH561, as well
o
L
+ 4kT R −R A + 4kTR
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.
f
o
v
o
Where the 4kT(R - R ) A term is the combined noise
power of R and R - R .
f
o
v
g
f
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
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.
of Figure 1, are shown below. Note that R may be
L
neglected in this calculation.
2
2
i
R −R
(
)
2
i
f
o
2
e =
e
+ i R
+ 4kTR +
+ L
(
)
n
ni
ni
s
s
2
A
v
4kT R −R
(
)
4kTR
f
o
o
+
2
A
A
v
v
10
REV. 1A January 2004
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
