OPA2690 datasheet(15/33 Pages) TI

15 / 33 page

OPA2822

SBOS188E

www.ti.com

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

309

Ω resistor on the noninverting input. The calculation for

this resistor value assumes a DC-coupled 50

Ω source

impedance along with RG and RM. While this resistor will

provide cancellation for the input bias current, it must be

well decoupled (0.1

µF in Figure 2) to filter the noise contri-

bution of the resistor itself and of the amplifier’s input

current noise.

As the required RG 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 50

Ω source impedance is

included in the analysis. For example, at a signal gain of

–12 (RG = 50Ω, RM = open, RF = 604Ω) the noise gain for the

circuit of Figure 2 will be 1 + 604

Ω/(50Ω + 50Ω) = 7, due to

the addition of the 50

Ω 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 2VPP output

capability on a single +5V supply, or 4VPP 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.

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 804

Ω 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 RF. 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

(57.6

Ω) 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 (RG) is AC-

coupled, giving a DC gain of +1. This centers the output also

at the input midpoint bias voltage (VS/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.

As with the circuit of Figure 2, the feedback resistor (RF) has

been increased to 604

Ω to reduce the loading effect it has

in parallel with the 100

Ω actual load. The noninverting input

is biased at VS/2 (2.5V in this case) using the two 1.21kΩ

resistors for RB. The parallel combination of these two

resistors (605

Ω) 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 0.1

µF capacitor to both reduce both

power-supply noise and the resistor and bias current noise

at this input.

FIGURE 3. AC-Coupled, G = +2, Single-Supply

Operation: Specification and Test Circuit.

FIGURE 4. AC-Coupled, G = –1, Single-Supply

Operation: Specification and Test Circuit.

1/2

OPA2822

+5V

S/2

100

Ω

804

Ω

804

Ω

57.6

Ω

402

Ω

402

Ω

0.1

µF

0.1

µF

6.8

µF

0.1

µF

1/2

OPA2822

+5V

S/2

100

Ω

1.21k

Ω

1.21k

Ω

0.1

µF

6.8

µF

54.9

Ω

Ω Source

604

Ω

604

Ω

0.1

µF

0.1

µF

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