OPA2846 datasheet(13/24 Pages) TI

13 / 24 page

OPA2846

SBOS274A −JUNE 2003 − REVISED MARCH 2004

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 2RG

equal to the required load looking out of the diplexer. The

signal gain is then set by adjusting feedback resistors, RF.

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.

1/ 2

OPA 28 46

1/ 2

OPA 28 46

Diplexer

Driver

Passive

Filter

Analog

Front

End

Low−Noise VDSL Receiver

Figure 3. Low-Noise VDSL Receiver

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 (CD) with the reverse bias

voltage (−VB) applied, the desired transimpedance gain

RF, and the GBP of the OPA2846 (1650MHz). Figure 4

shows a design using a 10pF source capacitance diode

and a 10k

Ω transimpedance gain. With these three

variables set (and including the parasitic input capacitance

for the OPA2846 added to CD), the feedback capacitor

value (CF) may be set to control the frequency response.

10k

Ω

Power−supply decoupling

not shown.

10pF

1/2

OP A28 46

+5V

−5V

−V

O =ID RF

0.3pF

Figure 4. Wideband, Low-Noise, Transimpedance

Amplifier

To achieve a maximally-flat, 2nd-order Butterworth

frequency response, the feedback pole should be set to:

1 (2

FCF)

+ GBP (4pR

FCD)

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

10k

Ω

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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