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AD834JR-REEL7 Datasheet(PDF) 5 Page - Analog Devices |
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AD834JR-REEL7 Datasheet(HTML) 5 Page - Analog Devices |
5 / 8 page AD834 REV. C –5– BASIC OPERATION Figure 7 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 (see Figure 7) which flows in the direction shown when X and Y are posi- tive. The outputs W1 and W2 each have a standing current of typically 8.5 mA. Figure 7. AD834 Functional Block Diagram The input voltages are first converted to differential currents which drive the translinear core. The equivalent resistance of the voltage-to-current (V-I) converters is about 285 Ω. 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 which provides a current gain of nominally × 1.6, trimmed during manufacture to set up the full-scale output cur- rent of ±4 mA. This output appears at a pair of open collectors which must be supplied with a voltage slightly above the voltage on Pin 6. As shown in Figure 8, 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 Ω, the voltage drop across it is about 600 mV. Using two 50 Ω load resistors, the full-scale differential output voltage is ±400 mV. The full bandwidth potential of the AD834 can only be realized when very careful attention is paid to grounding and decou- pling. 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 up- per 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 intro- duces 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, particularly when using higher supply voltages. Alterna- tively, lossy RF chokes or ferrite beads on the supply leads may be used. Figure 8 shows the use of optional termination resistors at the inputs. Note that although the resistive component of the input Figure 8. Basic Connections for Wideband Operation impedance is quite high (about 25 k Ω), 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 would be 25 Ω × 45 µA = 1.125 mV. This can be almost fully cancelled by including (in this example) another 25 Ω resistor in series with the “unused” input (in Figure 8, either X1 or Y2). In order 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: W = XY 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: W = XY 1V × 1 250 Ω 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 9 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 de- sired 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. |
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