OPA2690 datasheet(16/33 Pages) TI

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OPA2822

SBOS188E

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The gain resistor (RG) is set to equal the feedback resistor (RF)

at 604

Ω to achieve the desired gain of –1 from V

I to VO. A DC

blocking capacitor is included in series with RG to reduce the DC

gain for the noninverting input bias and offset voltages to +1.

This places the VS/2 bias voltage at the output pin and reduces

the output DC offset error terms. The signal input impedance is

matched to the 50

Ω source using the additional R

M resistor set

to 54.9

Ω. At higher frequencies, the parallel combination of R

and RG provides the input impedance match at 50Ω. This is

principally used for test and characterization purposes—system

applications do not necessarily require this input impedance

match, particularly if the source device is physically near the

OPA2822 and/or does not require a 50

Ω input impedance

match. At higher gains, the signal source impedance will start to

materially impact the apparent noise gain (and hence, band-

width) of the OPA2822.

ADSL RECEIVE AMPLIFIER

One of the principal applications for the OPA2822 is as a low-

power, low-noise receive amplifier in ADSL modem designs.

Applications ranging from single +5V,

±5V, and up to single +12V

supplies can be well supported by the OPA2822. For higher

supplies, consider the dual, low-noise THS6062 ADSL receive

amplifier that can support up to

±15V supplies. Figure 5 shows a

typical ADSL receiver design where the OPA2822 is used as an

inverting summing amplifier to provide both driver output signal

cancellation and receive channel gain. In the circuit of Figure 5,

the driver differential output voltage is shown as VD, while the

receiver channel output is shown as VR.

The two sets of resistors, R1 and R2, are set to provide the

desired gain from the transformer windings for the signal

arriving on the line side of the transformer, and also to provide

nominal cancellation for the driver output signal (VD) to the

receiver output. Typically, the two RS resistors are set to

provide impedance matching through the transformer. This is

accomplished by setting RS = 0.5 • (RL/N2), where N is the

turns ratio used for the line driver design. If RS is set in this

fashion, and the actual twisted pair line shows the expected RL

impedance value, the voltage swing produced at VD will be cut

in half at the transformer input. In this case, setting R1 = 2 • R2

will achieve cancellation of the driver output signal at the

output of the receiver. Essentially, the driver output voltage

produces a current in R1 that is exactly matched by the current

pulled out of R2 due to the attenuated and inverted version of

the output signal at the transformer input. In actual practice, R1

and R2 are usually RC networks to achieve cancellation over

the frequency varying line impedance.

As the transformer turns ratio changes to support different line

driver and supply voltage combinations, the impact of receiver

amplifier noise changes. Typically, DSL systems incur a line

referred noise contribution for the receiver that can be com-

puted for the circuit of Figure 5. For example, targeting an

overall gain of 1 from the line to the receiver output, and

picking the input resistor R2, the remaining resistors will be set

by the driver cancellation and gain requirements. With the

resistor values set, a line referred noise contribution due to the

OPA2822 can be computed. R1 will be set to 2x the value of

R2, and the feedback resistor will be set to recover the gain

loss through the transformer. Table I shows the total line

referred noise floor (in dBm/Hz) using three different values for

R2 over a range of transformer turns ratio (where the amplifier

gain is adjusted at each turns ratio).

Table I shows that a lower transformer turns ratio results in

reduced line referred noise, and that the resistor noise will

start to degrade the noise at higher values—particularly in

going from 500

Ω to 1kΩ. In general, line referred noise floor

due to the receiver channel will not be the limit to ADSL

modem performance, if it is lower than –145dBm.

FIGURE 5. Example ADSL Receiver Amplifier.

NR2 = 200

R2 = 500

R2 = 1000

–151.5

–150.2

–148.5

1.5

–149.1

–147.6

–145.8

–147.2

–145.6

–143.7

2.5

–145.6

–144.0

–142.1

–144.3

–142.7

–140.7

3.5

–143.2

–141.5

–139.5

–142.2

–140.5

–138.4

4.5

–141.3

–139.5

–137.5

–140.4

–138.7

–136.6

TABLE I. Line Referred Noise dBm/Hz, Due to Receiver

Op Amp.

1:n

Driver

1/2

OPA2822

+5V

1/2

OPA2822

–5V

Line

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