AD600ARZ-R7 datasheet(10/28 Pages) AD

10 / 28 page

AD600/AD602

Rev. E | Page 10 of 28

THEORY OF OPERATION

The AD600/AD602 have the same general design and features.

They comprise two fixed gain amplifiers, each preceded by a

voltage-controlled attenuator of 0 dB to 42.14 dB with independent

control interfaces, each having a scaling factor of 32 dB per volt.

The AD600 amplifiers are laser trimmed to a gain of 41.07 dB

(×113), providing a control range of −1.07 dB to +41.07 dB

(0 dB to +40 dB with overlap). The AD602 amplifiers have a

gain of 31.07 dB (×35.8) and provide an overall gain of

−11.07 dB to +31.07 dB (−10 dB to +30 dB with overlap).

The advantage of this topology is that the amplifier can use

negative feedback to increase the accuracy of its gain. In

addition, because the amplifier does not have to handle large

signals at its input, the distortion can be very low. Another

feature of this approach is that the small-signal gain and phase

response, and thus the pulse response, are essentially

independent of gain.

Figure 21 is a simplified schematic of one channel. The input

attenuator is a 7-stage R-2R ladder network, using untrimmed

resistors of nominally R = 62.5 Ω, which results in a characteristic

resistance of 125 Ω ± 20%. A shunt resistor is included at the

input and laser trimmed to establish a more exact input

resistance of 100 Ω ± 2%, which ensures accurate operation

(gain and HP corner frequency) when used in conjunction with

external resistors or capacitors.

PRECISION PASSIVE

INPUT ATTENUATOR

GATING

INTERFACE

SCALING

REFERENCE

GAT1

A1OP

A1CM

C1HI

C1LO

A1HI

A1LO

R-2R LADDER NETWORK

GAIN CONTROL

INTERFACE

RF2

2.24kΩ (AD600)

694Ω (AD602)

RF1

20Ω

FIXED-GAIN

AMPLIFIER

41.07dB (AD600)

31.07dB (AD602)

500Ω

0dB

–6.02dB

–12.04dB

–18.06dB

–22.08dB

–30.1dB

–36.12dB

–42.14dB

62.5Ω

Figure 21. Simplified Block Diagram of Single Channel of the AD600/AD602

The nominal maximum signal at input A1HI is 1 V rms (±1.4 V

peak) when using the recommended ±5 V supplies; although,

operation to ±2 V peak is permissible with some increase in HF

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.

The signal applied at the input of the ladder network is

attenuated by 6.02 dB by each section; thus, the attenuation to

each of the taps is progressively 0 dB, 6.02 dB, 12.04 dB, 18.06 dB,

24.08 dB, 30.1 dB, 36.12 dB, and 42.14 dB. A unique circuit

technique is employed to interpolate between these tap points,

indicated by the slider in Figure 21, providing continuous

attenuation from 0 dB to 42.14 dB.

To understand the AD600, it helps to think in terms of a

mechanical means for moving this slider from left to right; in

fact, it is voltage controlled. The details of the control interface

are discussed later. Note that the gain is exactly determined at

all times, and a linear decibel relationship is guaranteed

automatically between the gain and the control parameter that

determines the position of the slider. In practice, the gain

deviates from the ideal law by about ±0.2 dB peak (see Figure 28).

Note that the signal inputs are not fully differential. A1LO,

A1CM (for CH1), A2LO, and A2CM (for CH2) provide

separate access to the input and output grounds. This recognizes

that even when using a ground plane, small differences arise in the

voltages at these nodes. It is important that A1LO and A2LO be

connected directly to the input ground(s). Significant impedance in

these connections reduces the gain accuracy. A1CM and A2CM

should be connected to the load ground(s).

NOISE PERFORMANCE

An important reason for using this approach is the superior

noise performance that can be achieved. The nominal resistance

seen at the inner tap points of the attenuator is 41.7 Ω (one third of

125 Ω), which, at 27°C, exhibits a Johnson noise spectral density

(NSD) of 0.84 nV/√Hz (that is, √4kTR), a large fraction of the

total input noise. The first stage of the amplifier contributes

another 1.12 nV/√Hz, for a total input noise of 1.4 nV/√Hz.

The noise at the 0 dB tap depends on whether the input is

short-circuited or open-circuited. When shorted, the minimum

NSD of 1.12 nV/√Hz is achieved. When open, the resistance of

100 Ω at the first tap generates 1.29 nV/√Hz, so the noise

increases to 1.71 nV/√Hz. This last calculation would be important

if the AD600 were preceded, for example, by a 900 Ω resistor to

allow operation from inputs up to ±10 V rms. However, in most

cases, the low impedance of the source limits the maximum

noise resistance.

AD600ARZ-R7 数据表(PDF) 10 Page - Analog Devices

AD600ARZ-R7 数据表(HTML) 10 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

数据表下载