AD8260ACPZ-R7 datasheet(20/32 Pages) AD

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AD8260

Data Sheet

Rev. B | Page 20 of 32

THEORY OF OPERATION

OVERVIEW

The AD8260 is a self-contained transceiver intended for analog

communications using a power line as the media. Operating on

supplies as low as 3.3 V, it includes a high current driver usable

as a transmitter and a low noise digitally programmable variable

gain amplifier (DGA), usable as a receiver (see Figure 64). An

uncommitted current-feedback high frequency op amp acts as a

preamplifier and interface to the DGA and is user configured

for gains greater than 6 dB. Combined, the VGA and preamplifier

are usable at high signal levels from dc to 100 MHz, with a

small-signal −3 dB bandwidth of 230 MHz. To implement a

high current-output VGA, the VGA output can be connected

to the driver-amplifier differential input.

The small-signal −3 dB bandwidth of the driver amplifier is

195 MHz and the large-signal bandwidth is >115 MHz, even

when driving a 50 Ω load.

The device is fabricated on the Analog Devices, Inc., high speed

(eXtra Fast Complementary Bipolar) XFCB process. The pream-

plifier and DGA feature low dc offset voltage, and a nominal

gain range of −6 dB to +24 dB, a 30 dB gain span, and a differential

output for ADC driving. The power consumption is 93 mW

with a single 3.3 V supply. The supply current is typically about

28 mA when all circuits in the device are active. During normal

usage, either the driver amplifier is on or the preamplifier and

DGA are on and, therefore, the supply current in general is less

than 28 mA. The gain of the AD8260 VGA is programmed via a

4-bit parallel interface. Figure 64 shows the circuit block

diagram and basic application connections, and illustrates the

envisioned external DAC, ADC, and power-line bus interface

connections. The diagram shows the connections for single 3.3 V

supply operation; if a dual supply is available, the VMID

generator can be shut down and Pin VMDI, Pin VMDO, and

Pin VOCM need to be grounded. Note that Pin VNCM

functions as the negative supply for the bias and VMID cells,

plus the logic interfaces, and should always be tied to ground.

For optimal dynamic range, it is important that the inputs and

outputs to both the driver amplifier and the preamplifier and

the DGA output amplifier be ac-coupled in a single-supply

application. In Figure 64, the DAC and ADC are presumed to

operate on a 1.8 V or 3.3 V supply with a corresponding limited

output and input swing. The DAC outputs are currents that

point down and generate a voltage in the 50 Ω resistors that are

connected to ground. The maximum voltage with a peak DAC

output current of 15 mA is 0.75 V; if a DAC with a 20 mA peak

current is used, then the maximum voltage is 1 V per side for a

differential input signal of 2 V p-p.

The driver amplifier supports a 3 V p-p output swing on a

3.3 V supply. Because of its gain of 1.5, the maximum input

swing is 2 V p-p. The corresponding maximum output swing for

the DGA is 2.4 V p-p differential; the input to the preamplifier

can be a maximum of 0.6 V p-p.

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