Datasheet
4kT
R
G
R
G
R
F
R
S
OPA842
I
BI
E
O
I
BN
4kT = 16E 20J-
at 290 kelvins
E
RS
E
NI
4kTR
F
Ö
4kTR
S
Ö
E = E + (I R ) + 4kTR NG + (I R ) + 4kTR NG
O NI BN S S BI F F
2 2 2 2
(
(
E =E +(I R ) +4kTR +
N NI BN S S
2 2
4kTR
NG
F
(
(
I R
BI F
NG
2
+
NF = 10log
2 +
E
kTR
N
S
2
kT = 4E 21J at 290 kelvins-
OPA842
www.ti.com
SBOS267D –NOVEMBER 2002–REVISED SEPTEMBER 2010
approximately 600Hz. Starting from the –100dBc
second-harmonic for 2V
PP
into 200Ω, G = +2
distortion at 1MHz (from the Typical Characteristics),
the second-harmonic distortion at 20kHz should be
approximately:
–100dB – 20log (1MHz/20kHz) = –134dBc
The OPA842 has an extremely low third-order
harmonic distortion. This also gives an exceptionally
good two-tone, third-order intermodulation intercept,
as shown in the Typical Characteristics. This intercept
curve is defined at the 50Ω load when driven through
a 50Ω-matching resistor to allow direct comparisons
to RF MMIC devices. This network attenuates the
voltage swing from the output pin to the load by 6dB.
If the OPA842 drives directly into the input of a
Figure 43. Op Amp Noise Analysis Model
high-impedance device, such as an ADC, this 6dB
attenuation is not taken. Under these conditions, the
The total output spot noise voltage is computed as
intercept will increase by a minimum 6dBm. The
the square root of the squared contributing terms to
intercept is used to predict the intermodulation
the output noise voltage. This computation is adding
spurious for two closely spaced frequencies. If the
all the contributing noise powers at the output by
two test frequencies, f
1
and f
2
, are specified in terms
superposition, then taking the square root to get back
of average and delta frequency, f
O
= (f1 + f2)/2 and
to a spot noise voltage. Equation 3 shows the general
Δf = |f
2
– f
1
|/2, the two thirdorder, close-in spurious
form for this output noise voltage using the terms
tones will appear at f
O
± (3 • Δf). The difference
presented in Figure 43.
between the two equal test-tone power levels and
these intermodulation spurious power levels is given
by 2 • (IM
3
– P
O
), where IM
3
is the intercept taken
from the Typical Characteristic curve and P
O
is the
(3)
power level in dBm at the 50Ω load for one of the two
Dividing this expression by the noise gain [NG = (1 +
closely-spaced test frequencies. For instance, at
R
F
/R
G
)] will give the equivalent input-referred spot
10MHz, the OPA842 at a gain of +2 has an intercept
noise voltage at the noninverting input, as shown in
of 45dBm at a matched 50Ω load. If the full envelope
Equation 4.
of the two frequencies needs to be 2V
PP
, this requires
each tone to be 4dBm. The third-order
intermodulation spurious tones will then be
2 • (45 – 4) = 82dBc below the test-tone power level
(4)
(–80dBm). If this same 2V
PP
two-tone envelope were
delivered directly into the input of an ADC without the
Evaluating these two equations for the OPA842
matching loss or loading of the 50Ω network, the
circuit presented in Figure 37 will give a total output
intercept would increase to at least 51dBm. With the
spot noise voltage of 6.6nV/√Hz and an equivalent
same signal and gain conditions driving directly into a
input spot noise voltage of 3.3nV/√Hz.
light load, the spurious tones will then be at least
Narrow band communications systems are more
2 • (51 – 4) = 94dBc below the 1V
PP
test-tone signal
commonly concerned with the noise figure for the
levels.
amplifier. The total input referred voltage noise
expression (see Equation 4), may be used to
NOISE PERFORMANCE
calculate the noise figure. Equation 5 shows this
noise figure expression using the NG of Equation 4
The OPA842 complements its ultralow harmonic
for the noninverting configuration where the input
distortion with low input noise terms. Both the
terminating resistor, R
T
, has been set to match the
input-referred voltage noise and the two
source impedance, R
S
(see Figure 37).
input-referred current noise terms combine to give a
low output noise under a wide variety of operating
conditions. Figure 43 shows the op amp noise
analysis model with all the noise terms included. In
(5)
this model, all the noise terms are taken to be noise
voltage or current density terms in either nV/√Hz or
Evaluating Equation 5 for the circuit of Figure 37
pA/√Hz.
gives a noise figure = 17.6dB.
Copyright © 2002–2010, Texas Instruments Incorporated 17
Product Folder Link(s): OPA842