AD600ARZ-R7 datasheet(19/28 Pages) AD

19 / 28 page

AD600/AD602

Rev. E | Page 19 of 28

INPUT AMPLITUDE (V rms)

–0.4

+0.2

–0.2

0.001

0.01

0.1

100kHz

1MHz

10MHz

Figure 39. Output Stabilization vs. rms Input for

Sine Wave Inputs at 100 kHz, 1 MHz, and 10 MHz

While the band gap principle used here sets the output

amplitude to 1.2 V (for the square wave case), the stabilization

point can be set to any higher amplitude, up to the maximum

output of ±(VS − 2) V that the AD600 can support. It is only

necessary to split R2 into two components of appropriate ratio

whose parallel sum remains close to the zero-TC value of

806 Ω. Figure 40 shows this and how the output can be raised

without altering the temperature stability.

R2A

2N3904

VPTAT

OUTPUT

R2B

TO AD600 PIN 16

TO AD600 PIN 11

–

AD590

R2 = R2A || R2B ≈ 806Ω

300µA

(AT 300K)

1µF

15pF

Figure 40. Modification in Detector to Raise Output to 2 V rms

A WIDE RANGE, RMS-LINEAR dB MEASUREMENT

SYSTEM (2 MHz AGC AMPLIFIER WITH RMS

DETECTOR)

Monolithic rms-dc converters provide an inexpensive means to

measure the rms value of a signal of arbitrary waveform; they

can also provide a low accuracy logarithmic (decibel-scaled)

output. However, they have certain shortcomings. The first of

these is their restricted dynamic range, typically only 50 dB.

More troublesome is that the bandwidth is roughly proportional

to the signal level; for example, the AD636 provides a 3 dB

bandwidth of 900 kHz for an input of 100 mV rms but has a

bandwidth of only 100 kHz for a 10 mV rms input. Its

logarithmic output is unbuffered, uncalibrated, and not stable

over temperature. Considerable support circuitry, including at

least two adjustments and a special high TC resistor, is required

to provide a useful output.

These problems can be eliminated using an AD636 as the

detector element in an AGC loop, in which the difference

between the rms output of the amplifier and a fixed dc reference

are nulled in a loop integrator. The dynamic range and the

accuracy with which the signal can be determined are now

entirely dependent on the amplifier used in the AGC system.

Since the input to the rms-dc converter is forced to a constant

amplitude, close to its maximum input capability, the bandwidth is

no longer signal dependent. If the amplifier has an exactly

exponential (linear-dB) gain-control law, its control voltage VG

is forced by the AGC loop to have the general form

()

REF

RMS

SCALE

OUT

log

(4)

Figure 41 shows a practical wide dynamic range rms-

responding measurement system using the AD600. Note that

the signal output of this system is available at A2OP, and the

circuit can be used as a wideband AGC amplifier with an rms-

responding detector. This circuit can handle inputs from

100 μV to 1 V rms with a constant measurement bandwidth of

20 Hz to 2 MHz, limited primarily by the AD636 rms converter.

Its logarithmic output is a loadable voltage accurately calibrated

to 100 mV/dB or 2 V per decade, which simplifies the

interpretation of the reading when using a DVM and is

arranged to be −4 V for an input of 100 μV rms input, zero for

10 mV, and +4 V for a 1 V rms input. In terms of Equation 4,

VREF is 10 mV and VSCALE is 2 V.

Note that the peak log output of ±4 V requires the use of ±6 V

supplies for the dual op amp U3 (AD712) although lower

supplies would suffice for the AD600 and AD636. If only ±5 V

supplies are available, it is necessary to either use a reduced

value for VSCALE (say 1 V, in which case the peak output would

be only ±2 V) or restrict the dynamic range of the signal to

about 60 dB.

As in the previous case, the two amplifiers of the AD600 are

used in cascade. However, the 6 dB attenuator and low-pass

filter found in Figure 21 are replaced by a unity gain buffer

amplifier U3A, whose 4 MHz bandwidth eliminates the risk of

instability at the highest gains. The buffer also allows the use of

a high impedance coupling network (C1/R3) that introduces a

high-pass corner at about 12 Hz. An input attenuator of 10 dB

(X0.316) is now provided by R1 + R2 operating in conjunction

with the AD600’s input resistance of 100 Ω. The adjustment

provides exact calibration of the logarithmic intercept VREF in

critical applications, but R1 and R2 can be replaced by a fixed

resistor of 215 Ω if very close calibration is not needed, because

the input resistance of the AD600 (and all other key parameters

of it and the AD636) is already laser trimmed for accurate

operation. This attenuator allows inputs as large as ±4 V to be

accepted, that is, signals with an rms value of 1 V combined

with a crest factor of up to 4.

AD600ARZ-R7 数据表(PDF) 19 Page - Analog Devices

AD600ARZ-R7 数据表(HTML) 19 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