AD8260ACPZ-R7 datasheet(22/32 Pages) AD

22 / 32 page

AD8260

Data Sheet

Rev. B | Page 22 of 32

PRECAUTIONS TO BE OBSERVED DURING HALF-

DUPLEX OPERATION

During receive, when the high current driver-amplifier is

disabled, its gain setting resistors provide a signal path from

input to output. To prevent inadvertent DAC signals from being

transmitted while receiving via the preamplifier and DGA, the

DAC in Figure 64 must have no output signal.

During transmit, the preamplifier and VGA should be disabled

through any of the nongain-setting codes (see Table 4).

VMID BUFFER

The VMID buffer is a dc bias source that generates the voltage

on Pin 1 and Pin 19, VMDO. Node VMDO cannot accommodate

large dynamic currents and requires excellent ac decoupling to

ground. A high quality 0.1μF capacitor located as close as

possible to Pin 1 and Pin 19 (see Figure 64) is normally sufficient

to decouple the high values of current from Node VMDO.

When operating with dual power supplies, the buffer is disabled

by connecting Pin VMDI, Pin VOCM, and Pin VMDO to ground.

Because the logic decoder in the DGA (GNSx inputs) requires

3.3 V of headroom, the positive supply rails must be 3.3 V or

greater whether single-ended or dual. If a dual supply is used,

the negative rails are the same magnitude (opposite polarity)

as the positive, that is, −3.3 V when VPOS, VPSB, and VPSR

are +3.3 V.

PREAMPLIFIER

The AD8260 includes an uncommitted current feedback op

amp to buffer the resistive attenuator of the DGA. External

resistors are used to adjust the gain. The preamplifier is

characterized with a noninverting gain of 6 dB (2×) and both

gain resistor values of 100 Ω. The preamplifier gain can be

increased using different gain ratios of RFB1 and RFB2, trading off

bandwidth and offset voltage. The sum of the values of RFB1 and

RFB2 should be ≥200 Ω to maintain low distortion. RFB2 should

be ≥100 Ω because it and an internal compensation capacitor

determine the −3 dB bandwidth of the amplifier. Smaller

resistor values may compromise preamplifier stability.

Because the AD8260 is internally dc-coupled, larger preamplifier

gains increase its offset voltage. The circuit contains an internal

bias resistor and some offset compensation; however, if a lower

value of offset voltage is required, it can be compensated by

connecting a resistor between the FDBK pin and the supply

voltage. If the offset is negative, the resistor value connects to

the negative supply; otherwise, it connects to the positive supply.

For larger gains, the overall noise is reduced if a low value of

RFB1 is selected. For values of RFB1 = 20 Ω and RFB2 = 301 Ω, the

preamplifier gain is 16× (24.1 dB) and the input-referred noise

is about 1.5 nV/√Hz. For this value of gain, the overall gain range

increases by 18 dB so that the absolute gain range is 12 dB to 42 dB.

PREAMPLIFIER NOISE

The total input-referred voltage and current noise of the positive

input of the preamplifier is about 2.4 nV/Hz and 5 pA/Hz,

respectively. The DGA output referred noise is about 25 nV/Hz

at low gains and 39 nV/Hz at the highest gain. The 25 nV/Hz

divided by the DGA fixed gain of 8× results in 3.12 nV/Hz

referred to the DGA input. Note that this value includes the

noise of the DGA gain setting resistors as well. If this voltage is

divided by the preamplifier gain of 2×, the DGA noise referred

all the way to the preamplifier input is about 1.56 nV/Hz. From

this, it can be determined that the preamplifier, including the

100 Ω gain setting resistors, contributes about 1.8 nV/Hz. The

two 100 Ω resistors each contribute 1.29 nV/Hz at the output

of the preamplifier and 0.9 nV/Hz referred to the input. With

the gain resistor noise subtracted, the preamplifier noise alone

is about 1.6 nV/Hz.

Equation 1 shows the calculation that determines the output-

referred noise at maximum gain (24 dB or 16×).



)

(

)

(

)

(

)

(

)

(

VGA

RFB2

VGA

RFB1

PrA

out



























(1)

where:

At is the total gain from preamplifier input to the VGA output.

en,RS is the noise of the source resistance.

en,PrA is the input-referred voltage noise of the preamplifier.

in,PrA is the current noise of the preamplifier at the PRAI pin.

RS is the source resistance.

AVGA is the VGA gain.

en,RFB1 is the voltage noise of RFB1.

en,RFB2 is the voltage noise of RFB2.

en,VGA is the input-referred voltage noise of DGA (low gain output-referred noise divided by a fixed gain of 8×).

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