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AD834ARZ データシート(PDF) 11 Page - Analog Devices |
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AD834ARZ データシート(HTML) 11 Page - Analog Devices |
11 / 20 page Data Sheet AD834 Rev. F | Page 11 of 20 THEORY OF OPERATION Figure 11 is a functional equivalent of the AD834. There are three differential signal interfaces: the two voltage inputs (X = X1 − X2 and Y = Y1 − Y2), and the current output (W) which flows in the direction shown in Figure 11 when X and Y are positive. The outputs (W1 and W2) each have a standing current of typically 8.5 mA. X-DISTORTION CANCELLATION AD834 MULTIPLIER CORE CURRENT AMPLIFIER (W) ±4mA FS X2 X1 +VS W1 Y1 V-I V-I Y2 –VS W2 8.5mA 8.5mA 8 7 5 6 1 2 3 4 Y-DISTORTION CANCELLATION Figure 11. 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 Ω, which 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 manufacturing to set up the full-scale output current of ±4 mA. This output appears at a pair of open collec- tors that must be supplied with a voltage slightly above the voltage on Pin 6. As shown in Figure 12, this can be arranged by inserting a resistor in series with the supply to Pin 6 and taking the load resistors to the full supply. With R3 = 60 Ω, the voltage drop across it is about 600 mV. Using two 50Ω load resistors, the full-scale differential output voltage is ±400 mV. For best performance, the voltage on the output open-collectors (Pin 4 and Pin 5) must be higher than the voltage on Pin 6 by about 200 mV, as shown in Figure 12. 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 is usually determined by the care with which the layout is affected. 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, particu- larly when using higher supply voltages. Alternatively, lossy RF chokes or ferrite beads on the supply leads may be used. For best performance, use termination resistors at the inputs, as shown in Figure 12. Note that although the resistive component of the input impedance is quite high (about 25 kΩ), the input bias current of typically 45 μA can generate significant offset voltages if not compensated. For example, with a source and termination resistance of 50 Ω (net source of 25 Ω) the offset is 25 Ω × 45 μA = 1.125 mV. The offset can be almost fully cancelled by including (in this example) another 25 Ω resistor in series with the unused input. (In Figure 12, a 25 Ω resistor would be added from X1 to GND and Y2 to GND.) To minimize crosstalk, ground the input pins closest to the output (X1 and Y2); the effect is merely to reverse the phase of the X input and thus alter the polarity of the output. 8 7 6 5 1 2 3 4 X2 X1 +VS W1 Y1 Y2 –VS W2 AD834 X-INPUT ±1V FS Y-INPUT ±1V FS TERMINATION RESISTOR TERMINATION RESISTOR R3 62Ω R4 4.7Ω +5V –5V W OUTPUT ±400mV FS R1 49.9Ω R1 49.9Ω 1µF CERAMIC 1µF CERAMIC Figure 12. Basic Connections for Wideband Operation 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: ( ) mA 4 V 1 2 XY W = With the understanding that the inputs are specified in volts, the following simplified expression can be used: W = (XY)4 mA Alternatively, the full transfer function can be written as Ω 250 1 V 1 × = XY 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 become very high. |
同様の部品番号 - AD834ARZ |
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同様の説明 - AD834ARZ |
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