Analog Devices manual AD600/AD602, Rev. A

The emitter circuit of Q1 is somewhat inductive (due its finite ft and base resistance). Consequently, the effective value of R2 in- creases with frequency. This would result in an increase in the stabilized output amplitude at high frequencies, but for the ad- dition of C3, determined experimentally to be 15 pF for the 2N3904 for maximum response flatness. Alternatively, a faster transistor can be used here to reduce HF peaking. Figure 16 shows the ac response at the stabilized output level of about

1.3V rms. Figure 17 demonstrates the output stabilization for sine wave inputs of 1 mV to 1 V rms at frequencies of 100 kHz, 1 MHz and 10 MHz

OUTPUT CHANGE – dB	3dB
AGC

0.1110100

FREQUENCY – MHz

Figure 16. AC Response at the Stabilized Output Level of 1.3 V RMS

dB	+0.2
–
OUTPUT		1M	Hz
		100k	Hz
RELATIVE	0
	–0.4
	–0.2	10MHz

0.001 0.01 0.1 1

INPUT AMPLITUDE – Volts RMS

Figure 17. Output Stabilization vs. RMS Input for Sine Wave Inputs at 100 kHz, 1 MHz, and 10 MHz

While the “bandgap” principle used here sets the output ampli- tude 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 which the AD600 can support. It is only neces- sary to split R2 into two components of appropriate ratio whose parallel sum remains close to the zero-TC value of 806 Ω. This is illustrated in Figure 18, which shows how the output can be raised, without altering the temperature stability.

AD600/AD602

		+5V
	AD590	300∝A
		(at 300K)
TO AD600 PIN 16
C2
1∝F
		Q1
		2N3904
		R2B
C3	R2A VPTAT R2 = R2A R2B ≈ 806Ω
15pF

TO AD600 PIN 11	RF
	OUTPUT

Figure 18. Modification in Detector to Raise Output to 2 V RMS

AWide 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, and they also may 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 pro- portional 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.

All of these problems can be eliminated using an AD636 as merely the detector element in an AGC loop, in which the differ- ence between the rms output of the amplifier and a fixed dc ref- erence 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 band- width is no longer signal dependent. If the amplifier has an ex- actly exponential (“linear-dB”) gain-control law, its control voltage VG is forced by the AGC loop to be have the general form:

V	= V	log 10	VIN (RMS )	(4)
OUT	SCALE		VREF

Figure 19 shows a practical wide dynamic range rms-responding measurement system using the AD600. Note that the signal out- put of this system is available at A2OP, and the circuit can be used as a wideband AGC amplifier with an rms-responding de- tector. 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.

REV. A –11–