OPA2846 Datasheet, PDF (13/24 Pages) Texas Instruments – Dual, Wideband, Low-Noise, Voltage-Feedback Operational Amplifier

OPA2846
www.ti.com
LOW-NOISE VDSL RECEIVER
Most xDSL transceiver channels are differential for both
the driver and the receiver. The low-noise, high-gain
bandwidth, and low distortion for the dual OPA2846 make
it an ideal receiver channel element for the demanding
requirements emerging in VDSL. One possible imple-
mentation is shown in Figure 3. This circuit presumes full
duplex communication using frequency division multiplex-
ing, with send-and-receive isolation improved through the
use of a diplexer line interface. The differential receive
signal is brought into the inverting channel gain resistors
to get both noise and distortion improvement for a given
desired gain setting. To get impedance matching, set 2R_G
equal to the required load looking out of the diplexer. The
signal gain is then set by adjusting feedback resistors, R_F.
Using the OPA2846 in the inverting mode will give you a
reduced noise gain as described in the Wideband,
Inverting Gain Operation section of this data sheet. This
will improve both the SNR and distortion performance. If
the noise gain for a particular application drops below the
minimum recommended stable gain (+7), consider using
the Low-Gain Compensation technique described later in
this data sheet.
SBOS274A −JUNE 2003 − REVISED MARCH 2004
SINGLE-STAGE TRANSIMPEDANCE DESIGN
When setting up either one or both stages as a broadband
photodiode amplifier, the key elements in the design are
the expected diode capacitance (C_D) with the reverse bias
voltage (−V_B) applied, the desired transimpedance gain
R_F, and the GBP of the OPA2846 (1650MHz). Figure 4
shows a design using a 10pF source capacitance diode
and a 10kΩ transimpedance gain. With these three
variables set (and including the parasitic input capacitance
for the OPA2846 added to C_D), the feedback capacitor
value (C_F) may be set to control the frequency response.
+5V
Power−supply decoupling
not shown.
1/2
O P A28 46
V_O = I_D R_F
R_F
10kΩ
λ
I_D
C_D
10pF
C_F
−5V _0.3pF
1/2
OPA28 46
R_G
R_F
Passive ^Analog
R_G
R_F
Filter
Front
End
Diplexer
Driver
1/2
OPA28 46
Low−Noise VDSL Receiver
Figure 3. Low-Noise VDSL Receiver
−V_B
Figure 4. Wideband, Low-Noise, Transimpedance
Amplifier
To achieve a maximally-flat, 2nd-order Butterworth
frequency response, the feedback pole should be set to:
1ń(2pR_FC_F) + ^ǸǒGBPń(4pR_FC_D)^Ǔ
(1)
Adding the common-mode and differential mode input
capacitance (1.8 + 2.0)pF to the 10pF diode source
capacitance of Figure 4, and targeting a 10kΩ
transimpedance gain using the 1650MHz GBP for the
OPA2846, will require a feedback pole set to 31MHz. This
will require a total feedback capacitance of 0.5pF. Typical
surface-mount resistors have a parasitic capacitance of
0.2pF, leaving the required 0.3pF value shown in Figure 4
to get the required feedback pole.
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