OPA2822UG4 Datasheet, PDF (15/33 Pages) Texas Instruments – Dual, Wideband, Low-Noise Operational Amplifier

To take full advantage of the OPA2822’s excellent DC input
accuracy, the total DC impedance seen at of each of the
input terminals must be matched to get bias current cancel-
lation. For the circuit of Figure 2, this requires the grounded
309Ω resistor on the noninverting input. The calculation for
this resistor value assumes a DC-coupled 50Ω source
impedance along with R_G and R_M. While this resistor will
provide cancellation for the input bias current, it must be
well decoupled (0.1µF in Figure 2) to filter the noise contri-
bution of the resistor itself and of the amplifier’s input
current noise.
As the required R_G resistor approaches 50Ω at higher gains,
the bandwidth for the circuit in Figure 2 will far exceed the
bandwidth at the same gain magnitude for the noninverting
circuit of Figure 1. This occurs due to the lower noise gain for
the circuit of Figure 2 when the 50Ω source impedance is
included in the analysis. For example, at a signal gain of
–12 (R_G = 50Ω, R_M = open, R_F = 604Ω) the noise gain for the
circuit of Figure 2 will be 1 + 604Ω/(50Ω + 50Ω) = 7, due to
the addition of the 50Ω source in the noise gain equation.
This will give considerably higher bandwidth than the nonin-
verting gain of +12.
SINGLE-SUPPLY NONINVERTING OPERATION
The OPA2822 can also support single +5V operation with
its exceptional input and output voltage swing capability.
While not a rail-to-rail input/output design, both inputs and
outputs can swing to within 1.2V of either supply rail. For a
single amplifier channel, this gives a very clean 2V_PP output
capability on a single +5V supply, or 4V_PP output for a
differential configuration using both channels together. Fig-
ure 3 shows the AC-coupled noninverting gain of +2 used
as the basis of the Electrical Characteristics table and most
of the Typical Characteristics for single +5V supply opera-
tion.
+5V
+V_S
R_B
0.1µF
6.8µF
0.1µF
804Ω
V_I
57.6Ω
V_S/2
R_B
804Ω
1/2
OPA2822
R_L
V_O ^100Ω
V_S/2
R_F
402Ω
R_G
402Ω
0.1µF
FIGURE 3. AC-Coupled, G = +2, Single-Supply
Operation: Specification and Test Circuit.
The key requirement of broadband single-supply operation is
to maintain input and output signal swings within the usable
voltage range at both input and output. The circuit of Figure 3
establishes an input midpoint bias using a simple resistive
divider from the +5V supply (two 804Ω resistors). These two
resistors are selected to provide DC bias current cancellation
because their parallel combination matches the DC imped-
ance looking out of the inverting node, which equals R_F. The
gain setting resistor is not part of the DC impedance looking
out of the inverting node, due to the blocking capacitor in
series with it. The input signal is then AC-coupled into the
midpoint voltage bias. The input impedance matching resistor
(57.6Ω) is selected for testing to give a 50Ω input match (at
high frequencies) when the parallel combination of the biasing
divider network is included. The gain resistor (R_G) is AC-
coupled, giving a DC gain of +1. This centers the output also
at the input midpoint bias voltage (V_S/2). While this circuit is
shown using a +5V supply, this same circuit may be applied
for single-supply operation as high as +12V.
SINGLE-SUPPLY INVERTING OPERATION
For those single +5V Typical Characteristics that require
inverting gain of –1 operation, the test circuit in Figure 4 was
used.
+5V
+V_S
0.1µF
R_B
1.21kΩ
V_S/2
R_B
1.21kΩ
1/2
OPA2822
0.1µF
V_O
6.8µF
R_L
100Ω
V_S/2
50Ω Source
0.1µF
R_G
604Ω
V_I
R_M
54.9Ω
R_F
604Ω
FIGURE 4. AC-Coupled, G = –1, Single-Supply
Operation: Specification and Test Circuit.
As with the circuit of Figure 2, the feedback resistor (R_F) has
been increased to 604Ω to reduce the loading effect it has
in parallel with the 100Ω actual load. The noninverting input
is biased at V_S/2 (2.5V in this case) using the two 1.21kΩ
resistors for R_B. The parallel combination of these two
resistors (605Ω) provides input bias current cancellation by
matching the DC impedance looking out of the inverting
input node. The noninverting input bias is also well de-
coupled using the 0.1µF capacitor to both reduce both
power-supply noise and the resistor and bias current noise
at this input.
OPA2822
15
SBOS188E
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