OPA2822UG4 Datasheet, PDF (19/33 Pages) Texas Instruments – Dual, Wideband, Low-Noise Operational Amplifier

DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Two printed circuit boards (PCBs) are available to assist in
the initial evaluation of circuit performance using the OPA2822
in its two package options. Both of these are offered free of
charge as unpopulated PCBs, delivered with a user’s guide.
The summary information for these fixtures is shown in
Table III.
TABLE III. Demonstration Fixtures by Package.
PRODUCT
OPA2822U
OPA2822E
PACKAGE
SO-8
MSOP-8
ORDERING
NUMBER
DEM-OPA-SO-2A
DEM-OPA-MSOP-2A
LITERATURE
NUMBER
SBOU003
SBOU004
The demonstration fixtures can be requested at the Texas
Instruments web site (www.ti.com) through the OPA2822
product folder.
MACROMODELS AND APPLICATIONS SUPPORT
Computer simulation of circuit performance using SPICE is
often a quick way to analyze the performance of the OPA2822
in its intended application. This is particularly true for video
and RF amplifier circuits where parasitic capacitance and
inductance can play a major role in circuit performance. A
SPICE model for the OPA2822 is available through the TI
web site (www.ti.com). These models do a good job of
predicting small-signal AC and transient performance under
a wide variety of operating conditions. They do not do as well
in predicting the harmonic distortion characteristics. These
models do not attempt to distinguish between the package
types in their small-signal AC performance.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO MINIMIZE NOISE
Getting the full advantage of the OPA2822’s low input noise
requires careful attention to the external gain setting and DC
biasing networks. The feedback resistor is part of the overall
output load (which can begin to degrade distortion if set too
low). With this in mind, a good starting point for design is to
select the feedback resistor as low as possible (consistent
with loading distortion concerns), then continue with the
design, and set the other resistors as needed. To retain full
performance, setting the feedback resistor in the range of
200Ω to 750Ω can provide a good start to the design.
Figure 11 shows the full output noise analysis model for any
op amp.
The total output spot noise voltage can be computed as the
square root of the sum of all squared output noise voltage
terms. Equation 2 shows the general form of this output noise
voltage expression using the terms shown in Figure 11.
( ) ( ) ^{( ) E}O ⁼
ENI ²
+
I_BN R_S
2 _{+ 4kTRS}
NG²
+
I_BI R_F
2
+
4kTR_F
NG
(2)
E_NI
1/2
R_S
OPA2822
I_BN
E_O
E_RS
√4kTR_S
4kT
R_G
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6E –20J
at 290°K
FIGURE 11. Op Amp Noise Analysis Model.
Dividing this expression by the noise gain (NG = 1 = R_F/R_G)
will give the total equivalent spot noise voltage referred to the
noninverting input, as shown in Equation 3:
( ) E_N =
ENI2 +
I_BN R_S
2
+
4kTR_S
+

I_BI R_F
NG

2
+
4kTR_F
NG
(3)
Inserting high resistor values into Equation 3 can quickly
dominate the total equivalent input referred voltage noise. A
250Ω source impedance on the noninverting input will add as
much noise as the amplifier itself. If the noninverting input is
a DC bias path (as in inverting or in some single-supply
applications), it is critical to include a noise shunting capaci-
tor with that resistor to limit the added noise impact of those
resistors (see the example in Figure 2).
FREQUENCY RESPONSE CONTROL
Voltage-feedback op amps such as the OPA2822 exhibit
decreasing closed-loop bandwidth as the signal gain is
increased. In theory, this relationship is described by the
Gain Bandwidth Product (GBP) shown in the Electrical Char-
acteristics. Ideally, dividing GBP by the noninverting signal
gain (also called the Noise Gain, NG) will predict the closed-
loop bandwidth. In practice, this principle holds true only
when the phase margin approaches 90°, as it does in higher
gain configurations. At low gains, most high-speed amplifiers
will show a more complex response with lower phase margin
and higher bandwidth than predicted by the GBP. The
OPA2822 is compensated to give a slightly peaked fre-
quency response at a gain of +2 (see the circuit in Figure 1).
The 200MHz typical bandwidth at a gain of +2 far exceeds
that predicted by dividing the GBP of 240MHz by a gain of 2.
The bandwidth predicted by the GBP is more closely correct
as the gain increases. As shown in the Typical Characteris-
tics, at a gain of +10, the –3dB bandwidth of 24MHz matches
that predicted by dividing the GBP by 10.
OPA2822
19
SBOS188E
www.ti.com