AD8366ACPZ-R7 datasheet(15/28 Pages) AD

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Data Sheet

AD8366

Rev. B | Page 15 of 28

CIRCUIT DESCRIPTION

The AD8366 is a dual, differential, digitally controlled VGA

with 600 MHz of 3 dB bandwidth and a gain range of 4.5 dB to

20.25 dB adjustable in 0.25 dB steps. Using a proprietary variable

gain architecture, the AD8366 is able to achieve excellent linearity

(45 dBm) and noise performance (11.7 nV/√Hz) at 10 MHz at

minimum gain. Intended for use in direct conversion systems, the

part also includes dc offset correction that can be disabled easily

by grounding either OFSA or OFSB. In addition, the part offers

an adjustable output common-mode range of 1.6 V to 3 V.

The main signal path is shown in Figure 46. It consists of an

input transconductance, a variable-gain cell, and an output

transimpedance amplifier.

100Ω

12.5Ω

VARIABLE

CURRENT-GAIN

STAGE

OUTPUT

BUFFER

VIRTUAL

GROUND

VIRTUAL

GROUND

INP

INM

OUTP

OUTM

Figure 46. Main Signal Path

The input transconductance provides a broadband 200 Ω

differential termination and converts the input voltage to a

current. This current is fed into the variable current-gain cell.

The output of this cell goes into the transimpedance stage, which

generates the output voltage. The transimpedance is fixed at 500 Ω,

with a roughly 25 Ω differential output impedance.

INPUTS

The inputs to the digitally-controlled VGAs in the AD8366 are

differential and can be either ac- or dc-coupled. The AD8366

synthesizes a 200 Ω (differential) input impedance, with a return

loss (re: 200 Ω) of better than 10 dB to 200 MHz. The nominal

common-mode input voltage to the part is VPOS/2, but the AD8366

can be dc-coupled to parts with lower common modes if these

parts can sink current. The amount of current sinking required

depends on the input common-mode level and is given by

ISINK

(per leg) = (VPOS/2 − VICM)/100

The input common-mode range is 1.5 V to VPOS/2.

OUTPUTS

The outputs of the digitally-controlled VGAs are differential and

can be either ac- or dc-coupled. The AD8366 synthesizes a 25 Ω

differential output impedance, with a return loss (re: 25 Ω) of

better than 10 dB to 120 MHz. The nominal common-mode

output voltage is VPOS/2; however, it can be lowered or raised by

driving the VCMA or VCMB pins.

OUTPUT DIFFERENTIAL OFFSET CORRECTION

To prevent significant levels of offset from appearing at the

outputs of the AD8366, each digitally controlled VGA has a

differential offset correction loop, as shown in Figure 47. This

loop senses any differential offset at the output and corrects for

it by injecting an opposing current at the input differential ground.

The loop is able to correct for input dc offsets of up to ±20 mV.

Because the loop automatically nulls out any dc or low frequency

offset, the effect of the loop is to introduce a high-pass corner into

the transfer function of the digitally controlled VGA. The

location of this high-pass corner depends on both the gain

setting and the value of the capacitor connected to the OFSx pin

(OFSA for DVGA A and OFSB for DVGA B) and is given by

(

)

(

)

(

)

2π

4000

1.037

4300

kHz

OFS

where:

GC is the gain code (a value from 0 to 63).

COFS

is the value of the capacitance connected to OFSA or OFSB,

in picofarads (pF).

The offset correction loop can be disabled by grounding either

OFSA or OFSB.

gm1

gm2

INP

INM

OFFSET

COMPENSATION

LOOP

VARIABLE-GAIN

STAGE

OUTPUT

BUFFER

OUTP

OUTM

COFS

Figure 47. Differential Offset Correction Loop

OUTPUT COMMON-MODE CONTROL

To interface to ADCs that require different input common-mode

voltages, the AD8366 has an adjustable output common-mode

level. The output common-mode level is normally set to VPOS/2;

however, it can be changed between 1.6 V and 3 V by driving

the VCMA pin or the VCMB pin. The input equivalent circuit

for the VCMA pin is shown in Figure 48; the VCMB pin has the

same input equivalent circuit.

4kΩ

500Ω

VPOS/2

VCMA

Figure 48. Input Equivalent Circuit for VCMA

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