OPA2822UG4 Datasheet, PDF (16/33 Pages) Texas Instruments – Dual, Wideband, Low-Noise Operational Amplifier

The gain resistor (R_G) is set to equal the feedback resistor (R_F)
at 604Ω to achieve the desired gain of –1 from V_I to V_O. A DC
blocking capacitor is included in series with R_G to reduce the DC
gain for the noninverting input bias and offset voltages to +1.
This places the V_S/2 bias voltage at the output pin and reduces
the output DC offset error terms. The signal input impedance is
matched to the 50Ω source using the additional R_M resistor set
to 54.9Ω. At higher frequencies, the parallel combination of R_M
and R_G 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 50Ω 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 V_D, while the
receiver channel output is shown as V_R.
+5V
Driver
R_S
R₂
1/2
OPA2822
R_F
R₁
1:n
V_D
R_L
Line
V_R
R₁
R_S
R₂
R_F
1/2
OPA2822
–5V
FIGURE 5. Example ADSL Receiver Amplifier.
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 (V_D) to the
receiver output. Typically, the two R_S resistors are set to
provide impedance matching through the transformer. This is
accomplished by setting R_S = 0.5 • (R_L/N²), where N is the
turns ratio used for the line driver design. If R_S is set in this
fashion, and the actual twisted pair line shows the expected R_L
impedance value, the voltage swing produced at V_D 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. Line Referred Noise dBm/Hz, Due to Receiver
Op Amp.
N
R₂ = 200
R₂ = 500
R₂ = 1000
1
–151.5
–150.2
–148.5
1.5
–149.1
–147.6
–145.8
2
–147.2
–145.6
–143.7
2.5
–145.6
–144.0
–142.1
3
–144.3
–142.7
–140.7
3.5
–143.2
–141.5
–139.5
4
–142.2
–140.5
–138.4
4.5
–141.3
–139.5
–137.5
5
–140.4
–138.7
–136.6
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 500Ω 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.
16
OPA2822
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