AD8331 datasheet(17/32 Pages) AD

17 / 32 page

AD8331/AD8332

Rev. C | Page 17 of 32

THEORY OF OPERATION

OVERVIEW

The following discussion applies to all part numbers. Figure 56

and Figure 1 are functional block diagrams of the AD8331 and

AD8332, respectively.

LNA

LMD

INH

COML

VPSL

GAIN

COMM

LNA

BIAS

(VMID)

VGA

∆G = –48dB to 0dB

+21dB

BIAS AND

INTERPOLATOR

VMID

POST

AMP1

CLAMP

GAIN

INT

3.5dB/

15.5dB

ENBL

ENBV

RCLMP

VOH

VOL

MODE

LON LOP VIP VIN

VPOS

VCM

HILO

COMM

AD8331

Figure 56. Functional Block Diagram — AD8331

Each channel contains an LNA that provides user-adjustable

input impedance termination, a differential X-AMP VGA, and a

programmable gain postamplifier with adjustable output

voltage limiting. Figure 57 shows a simplified block diagram.

VOL

VOH

LNA

CLAMP*

–

INH

LMD

LOP

LON

HILO

VCM

PREAMPLIFIER

19dB

X-AMP VGA

POSTAMP

[(–48 to 0) + 21] dB

3.5dB/15.5dB

GAIN

INTERFACE*

GAIN

BIAS AND

INTERPOLATOR*

VIN

VIP

*SHARED BETWEEN CHANNELS

RCLMP

BIAS

(VMID)

VMID

Figure 57. Simplified Block Diagram

The linear-in-dB gain control interface is trimmed for slope and

absolute accuracy. The overall gain range is 48 dB, extending

from –4.5 dB to +43.5 dB or from +7.5 dB to +55.5 dB,

depending on the setting of the HILO pin. The slope of the gain

control interface is 50 dB/V, and the gain control range is 40 mV

to 1 V, leading to the following expressions for gain:

(

)

() ( )

)

(

HILO

–

GAIN

(

)

() ( )

)

(

HILO

GAIN

The gain characteristics are shown in Figure 58.

00.2

VGAIN (V)

0.6

0.4

–10

1.0

0.8

1.1

HILO = HI

HILO = LO

MODE = LO

MODE = HI

(WHERE AVAILABLE)

Figure 58. Gain Control Characteristics

When MODE is set high, (where available):

(

)

() ()

–

)

(

HILO

GAIN

(

)

() ()

–

)

(

HILO

GAIN

The LNA converts a single-ended input to a differential output

with a voltage gain of 19 dB. When only one output is used, the

gain is 13 dB. The inverting output is used for active input

impedance termination. Each of the LNA outputs is capacitively

coupled to a VGA input. The VGA consists of an attenuator

with a range of 48 dB followed by an amplifier with 21 dB of

gain, for a net gain range of –27 dB to +21 dB. The X-AMP

gain-interpolation technique results in low gain error and

uniform bandwidth, and differential signal paths minimize

distortion.

The final stage is a logic programmable amplifier with gains of

3.5 dB or 15.5 dB. The LO and HI gain modes are optimized

for 12-bit and 10-bit A/D converter applications, in terms of

output-referred noise and absolute gain range. Output voltage

limiting may be programmed by the user.

LOW NOISE AMPLIFIER (LNA)

Good noise performance relies on a proprietary ultralow noise

preamplifier at the beginning of the signal chain, which

minimizes the noise contribution in the following VGA. Active

impedance control optimizes noise performance for

applications that benefit from input matching.

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