AD834AR 查看數據表（PDF） - Analog Devices

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AD834AR

500 MHz Four-Quadrant Multiplier

Analog Devices 

AD834

BASIC OPERATION

Figure 4 is a functional equivalent of the AD834. There are three

differential signal interfaces: the voltage inputs X = X1–X2 and

Y = Y1–Y2, and the current output, W, which flows in the

direction shown when X and Y are positive. The outputs W1

and W2 each have a standing current of typically 8.5 mA.

X2

X1

+VS

W1

8

7

6

5

V/I

AD834

8.5mA

X-DISTORTION

CANCELLATION

MULTIPLIER CORE

Y-DISTORTION

CANCELLATION

CURRENT

AMPLIFIER

(W)

؎4mA

FS

8.5mA

V/I

1

2

3

4

Y1

Y2

–VS

W2

Figure 4. Functional Block Diagram

The input voltages are first converted to differential currents

that drive the translinear core. The equivalent resistance of the

voltage-to-current (V-I) converters is about 285 W. This low

value results in low input related noise and drift. However, the

low full-scale input voltage results in relatively high nonlinearity

in the V-I converters. This is significantly reduced by the use of

distortion cancellation circuits, which operate by Kelvin sensing

the voltages generated in the core—an important feature of

the AD834.

The current mode output of the core is amplified by a special

cascode stage that provides a current gain of nominally ¥ 1.6,

trimmed during manufacture to set up the full-scale output

current of ± 4 mA. This output appears at a pair of open collectors

that must be supplied with a voltage slightly above the voltage on

Pin 6. As shown in Figure 5, this can be arranged by inserting a

resistor in series with the supply to this pin and taking the load

resistors to the full supply. With R3 = 60 W, the voltage drop

across it is about 600 mV. Using two 50 W load resistors, the

full-scale differential output voltage is ± 400 mV.

The full bandwidth potential of the AD834 can be realized only

when very careful attention is paid to grounding and decoupling.

The device must be mounted close to a high quality ground

plane and all lead lengths must be extremely short, in keeping

with UHF circuit layout practice. In fact, the AD834 shows

useful response to well beyond 1 GHz, and the actual upper

frequency in a typical application will usually be determined by

the care with which the layout is effected. Note that R4 (in series

with the –VS supply) carries about 30 mA and thus introduces a

voltage drop of about 150 mV. It is made large enough to reduce

the Q of the resonant circuit formed by the supply lead and the

decoupling capacitor. Slightly larger values can be used, par-

ticularly when using higher supply voltages. Alternatively, lossy

RF chokes or ferrite beads on the supply leads may be used.

Figure 5 shows the use of optional termination resistors at the

inputs. Note that although the resistive component of the input

X-INPUT

؎1V FS

R3

62⍀

OPTIONAL

TERMINATION

RESISTOR

1␮F

CERAMIC

+5V

R1

R2

49.9⍀ 49.9⍀

8765

X2 X1 +VS W1

AD834

Y1 Y2 –VS W2

1234

W OUTPUT

؎400mV FS

Y-INPUT

؎1V FS

OPTIONAL

TERMINATION

RESISTOR

1␮F

CERAMIC

R4

4.7⍀

–5V

Figure 5. Basic Connections for Wideband Operation

impedance is quite high (about 25 kW), the input bias current

of typically 45 mA can generate significant offset voltages if not

compensated. For example, with a source and termination

resistance of 50 W (net source of 25 W) the offset would be

25 W ¥ 45 mA = 1.125 mV. This can be almost fully cancelled by

including (in this example) another 25 W resistor in series with

the “unused” input (in Figure 5, either X1 or Y2). To minimize

crosstalk, the input pins closest to the output (X1 and Y2) should

be grounded; the effect is merely to reverse the phase of the X

input and thus alter the polarity of the output.

TRANSFER FUNCTION

The output current W is the linear product of input voltages

X and Y divided by (1 V)2 and multiplied by the “scaling

current” of 4 mA:

XY

W = (1V )2 4 mA

Provided that it is understood that the inputs are specified in

volts, a simplified expression can be used:

W = (XY ) 4 mA

Alternatively, the full transfer function can be written:

XY 1

W = 1V ¥ 250 W

When both inputs are driven to their clipping level of about

1.3 V, the peak output current is roughly doubled to ± 8 mA,

but distortion levels will then be very high.

TRANSFORMER COUPLING

In many high-frequency applications where baseband operation

is not required at either inputs or output, transformer coupling

can be used. Figure 6 shows the use of a center-tapped output

transformer, which provides the necessary dc load condition at

the outputs W1 and W2 and is designed to match into the desired

load impedance by appropriate choice of turns ratio. The specific

choice of the transformer design will depend entirely on the

application. Transformers may also be used at the inputs.

Center-tapped transformers can reduce high frequency distor-

tion and lower HF feedthrough by driving the inputs with

balanced signals.

–6–

REV. D

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