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CLC425AJE Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part # CLC425AJE
Description  Ultra Low Noise Wideband Op Amp
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

CLC425AJE Datasheet(HTML) 6 Page - National Semiconductor (TI)

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As seen in Figure 5, eni is dominated by the intrinsic
voltage noise (en) of the amplifier for equivalent source
resistances below 33.5
Ω. Between 33.5Ω and 6.43kΩ,
eni is dominated by the thermal noise (e
4TR
t
seq
=
k
) of
the external resistors. Above 6.43k
Ω, e
ni is dominated by
the amplifier's current noise ( 2i R
n
seq ). The point at
which the CLC425's voltage noise and current noise
contribute equally occurs for Rs
eq=464Ω (i.e. e2i
nn
/
).
As an example, configured with a gain of +20V/V giving
a -3dB of 90MHz and driven from an Rs
eq=25Ω, the
CLC425 produces a total equivalent input noise voltage
(e
1.57 90MHz
ni ∗∗
) of 16.5
µV
rms.
Figure 5: Voltage Noise Density vs. Source Resistance
If bias current cancellation is not a requirement, then
Rf||Rg does not need to equal Rs
eq. In this case, according
to Equation 1, Rf||Rg should be as low as possible in
order to minimize noise. Results similar to Equation 1
are obtained for the inverting configuration of Figure 2 if
Rs
eq is replaced by Rb and Rg is replaced by Rg+Rs. With
these substitutions, Equation 1 will yield an eni refered to
the non-inverting input. Refering eni to the inverting input
is easily accomplished by multiplying eni by the ratio of
non-inverting to inverting gains.
Noise Figure
Noise Figure (NF) is a measure of the noise degradation
caused by an amplifier.
The Noise Figure formula is shown in Equation 3. The
addition of a terminating resistor RT,
reduces the
external thermal noise but increases the resulting NF.
The NF is increased because RT reduces the input signal
amplitude thus reducing the input SNR.
Rseq = Rs for Unterminated Systems
Rseq = Rs II RT for Terminated Systems
Equation 3: Noise Figure Equation
The noise figure is related to the equivalent source
resistance (Rs
eq) and the parallel combination of Rf and
Rg. To minimize noise figure, the following steps are
recommended:
Minimize R
f||Rg
Choose the optimum R
s (ROPT)
ROPT is the point at which the NF curve reaches a
minimum and is approximated by:
ROPT
≅ (e
n/in)
Figure 6 is a plot of NF vs Rs with Rf||Rg = 9.09 (Av = +10).
The NF curves for both Unterminated and Terminated
systems are shown. The Terminated curve assumes Rs
= RT. The table indicates the NF for various source
resistances including Rs = ROPT.
Figure 6: Noise Figure vs Source Resistance
Supply Current Adjustment
The CLC425's supply current can be externally adjusted
downward from its nominal value by adding an optional
resistor (Rp) between pin 8 and the negative supply as
shown in Figure 7. Several of the plots found within the plot
pages demonstrate the CLC425’s behavior at different
supply currents. The plot labeled “Icc vs. Rp” provides the
means for selecting Rp and shows the result of standard IC
process variation which is bounded by the 25°C curve.
Figure 7: External Supply Current Adjustment
Non-Inverting Gains Less Than 10V/V
Using the CLC425 at lower non-inverting gains requires
external compensation such as the shunt compensation
as shown in Figure 8. The quiescent supply current must
also be reduced to 5mA with Rp for stability. The com-
pensation capacitors are chosen to reduce frequency
response peaking to less than 1dB. The plot in the
"Typical Performance" section labeled “Differential Gain
and Phase” shows the video performance of the CLC425
with this compensation circuitry.
3
2
4
7
6
8
+Vcc
-Vcc
Vout
Rp
CLC425
NF
10LOG
S/ N
S/ N
10LOG
e
e
ii
oo
ni
2
t
2
=


=


NF
10LOG
e
i
R
R | | R
4kTR
4kT R | | R
4kTR
n
2
n
2
seq
f
g
2
seq
f
g
seq
=
++
()
 ++
()
http://www.national.com
6


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