Datasheet AD600, AD602 (Analog Devices) - 10

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AD600/AD602 THEORY OF OPERATION
The AD600/AD602 have the same general design and features. The signal applied at the input of the ladder network is They comprise two fixed gain amplifiers, each preceded by a attenuated by 6.02 dB by each section; thus, the attenuation to voltage-controlled attenuator of 0 dB to 42.14 dB with independent each of the taps is progressively 0 dB, 6.02 dB, 12.04 dB, 18.06 dB, control interfaces, each having a scaling factor of 32 dB per volt. 24.08 dB, 30.1 dB, 36.12 dB, and 42.14 dB. A unique circuit The AD600 amplifiers are laser trimmed to a gain of 41.07 dB technique is employed to interpolate between these tap points, (×113), providing a control range of −1.07 dB to +41.07 dB indicated by the slider in Figure 21, providing continuous (0 dB to +40 dB with overlap). The AD602 amplifiers have a gain attenuation from 0 dB to 42.14 dB. of 31.07 dB (×35.8) and provide an overall gain of −11.07 dB to To understand the AD600/AD602, it helps to think in terms of +31.07 dB (−10 dB to +30 dB with overlap). a mechanical means for moving this slider from left to right; in The advantage of this topology is that the amplifier can use fact, it is voltage controlled. The details of the control interface negative feedback to increase the accuracy of its gain. In are discussed later. Note that the gain is exactly determined at addition, because the amplifier does not have to handle large all times and a linear decibel relationship is guaranteed auto- signals at its input, the distortion can be very low. Another matically between the gain and the control parameter that feature of this approach is that the small-signal gain and phase determines the position of the slider. In practice, the gain response, and thus the pulse response, are essentially deviates from the ideal law by about ±0.2 dB peak (see Figure 28). independent of gain. Note that the signal inputs are not fully differential. A1LO, A1CM Figure 21 is a simplified schematic of one channel. The input (for CH1), A2LO, and A2CM (for CH2) provide separate access attenuator is a 7-stage R-2R ladder network, using untrimmed to the input and output grounds. This recognizes that, even when resistors of nominally R = 62.5 Ω, which results in a characteristic using a ground plane, small differences arise in the voltages at resistance of 125 Ω ± 20%. A shunt resistor is included at the these nodes. It is important that A1LO and A2LO be connected input and laser trimmed to establish a more exact input directly to the input ground(s). Significant impedance in these resistance of 100 Ω ± 2%, which ensures accurate operation connections reduces the gain accuracy. A1CM and A2CM (gain and HP corner frequency) when used in conjunction with should be connected to the load ground(s). external resistors or capacitors.
NOISE PERFORMANCE GAT1
An important reason for using this approach is the superior
PRECISION PASSIVE SCALING GATING
noise performance that can be achieved. The nominal resistance
INPUT ATTENUATOR REFERENCE INTERFACE
seen at the inner tap points of the attenuator is 41.7 Ω (one third of
C1HI
125 Ω), which, at 27°C, exhibits a Johnson noise spectral density
VG A1OP C1LO
(NSD) of 0.84 nV/√Hz (that is, √4kTR), a large fraction of the
A1CM GAIN CONTROL
total input noise. The first stage of the amplifier contributes
INTERFACE RF2
another 1.12 nV/√Hz, for a total input noise of 1.4 nV/√Hz.
2.24kΩ (AD600) 0dB –12.04dB –22.08dB –36.12dB 694Ω (AD602)
The noise at the 0 dB tap depends on whether the input is
–6.02dB –18.06dB –30.1dB –42.14dB RF1 A1HI 20Ω
short-circuited or open-circuited. When shorted, the minimum
FIXED-GAIN
NSD of 1.12 nV/√Hz is achieved. When open, the resistance of
A1LO AMPLIFIER
1
500Ω R-2R LADDER NETWORK 62.5Ω 41.07dB (AD600)
02 8- 100 Ω at the first tap generates 1.29 nV/√Hz, so the noise
31.07dB (AD602)
53 00 increases to 1.71 nV/√Hz. This last calculation would be important Figure 21. Simplified Block Diagram of a Single Channel of the AD600/AD602 if the AD600 were preceded, for example, by a 900 Ω resistor to The nominal maximum signal at input A1HI is 1 V rms (±1.4 V allow operation from inputs up to ±10 V rms. However, in most peak) when using the recommended ±5 V supplies, although cases, the low impedance of the source limits the maximum operation to ±2 V peak is permissible with some increase in HF noise resistance. distortion and feedthrough. Each attenuator is provided with a separate signal LO connection for use in rejecting common mode, the voltage between input and output grounds. Circuitry is included to provide rejection of up to ±100 mV. Rev. F | Page 10 of 32 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION NOISE PERFORMANCE GAIN-CONTROL INTERFACE SIGNAL-GATING INPUTS COMMON-MODE REJECTION ACHIEVING 80 dB GAIN RANGE SEQUENTIAL MODE (MAXIMUM SNR) PARALLEL MODE (SIMPLEST GAIN-CONTROL INTERFACE) LOW RIPPLE MODE (MINIMUM GAIN ERROR) APPLICATIONS INFORMATION TIME-GAIN CONTROL (TGC) AND TIME-VARIABLE GAIN (TVG) INCREASING OUTPUT DRIVE DRIVING CAPACITIVE LOADS REALIZING OTHER GAIN RANGES ULTRALOW NOISE VCA LOW NOISE, 6 dB PREAMPLIFIER LOW NOISE AGC AMPLIFIER WITH 80 dB GAIN RANGE WIDE RANGE, RMS-LINEAR dB MEASUREMENT SYSTEM (2 MHz AGC AMPLIFIER WITH RMS DETECTOR) 100 dB TO 120 dB RMS RESPONDING CONSTANT BANDWIDTH AGC SYSTEMS WITH HIGH ACCURACY DECIBEL OUTPUTS 100 dB RMS/AGC SYSTEM WITH MINIMAL GAIN ERROR (PARALLEL GAIN WITH OFFSET) 120 dB RMS/AGC SYSTEM WITH OPTIMAL SNR (SEQUENTIAL GAIN) OUTLINE DIMENSIONS ORDERING GUIDE