AD603 datasheet(20/25 Pages) AD

20 / 25 page

Data Sheet

AD603

Rev. K | Page 19 of 24

During the time VOUT is negative with respect to the base

voltage of Q1, Q1 conducts; when VOUT is positive, it is cut off.

Because the average collector current of Q1 is forced to be

300 µA, and the square wave has a duty cycle of 1:1, Q1’s

collector current when conducting must be 600 µA. With R8

omitted, the peak amplitude of VOUT is forced to be just the VBE

of Q1 at 600 µA, typically about 700 mV, or 2 VBE peak-to-peak.

This voltage, the amplitude at which the output stabilizes, has a

strong negative temperature coefficient (TC), typically −1.7 mV/°C.

Although this may not be troublesome in some applications, the

correct value of R8 renders the output stable with temperature.

To understand this, note that the current in Q2 is made to be

proportional to absolute temperature (PTAT). For the moment,

continue to assume that the signal is a square wave.

When Q1 is conducting, VOUT is now the sum of VBE and a

voltage that is PTAT and that can be chosen to have an equal

but opposite TC to that of the VBE. This is actually nothing more

than an application of the band gap voltage reference principle.

When R8 is chosen such that the sum of the voltage across it

and the VBE of Q1 is close to the band gap voltage of about 1.2 V,

VOUT is stable over a wide range of temperatures, provided, of

course, that Q1 and Q2 share the same thermal environment.

Because the average emitter current is 600 µA during each half

cycle of the square wave, a resistor of 833 Ω adds a PTAT

voltage of 500 mV at 300 K, increasing by 1.66 mV/°C. In

practice, the optimum value depends on the type of transistor

used and, to a lesser extent, on the waveform for which the

temperature stability is to be optimized; for the inexpensive

2N3904/2N3906 pair and sine wave signals, the recommended

value is 806 Ω.

This resistor also serves to lower the peak current in Q1 when

more typical signals (usually sinusoidal) are involved, and the

1.8 kHz LP filter it forms with CAV helps to minimize distortion

due to ripple in VAGC. Note that the output amplitude under sine

wave conditions is higher than for a square wave because the

average value of the current for an ideal rectifier is 0.637 times

as large, causing the output amplitude to be 1.88 (= 1.2/0.637) V,

or 1.33 V rms. In practice, the somewhat nonideal rectifier

results in the sine-wave output being regulated to about

1.4 V rms, or 3.6 V p-p.

The bandwidth of the circuit exceeds 40 MHz. At 10.7 MHz, the

AGC threshold is 100 µV (−67 dBm) and its maximum gain is

83 dB (20 log 1.4 V/100 µV). The circuit holds its output at

1.4 V rms for inputs as low as −67 dBm to +15 dBm (82 dB),

where the input signal exceeds the maximum input rating of the

AD603. For a 30 dBm input at 10.7 MHz, the second harmonic

is 34 dB down from the fundamental, and the third harmonic is

35 dB down from the fundamental.

CAUTION

Careful component selection, circuit layout, power supply

decoupling, and shielding are needed to minimize the susceptibility

of the AD603 to interference from signals such as those from

radio and TV stations. In bench evaluation, it is recommended

to place all of the components into a shielded box and use

feedthrough decoupling networks for the supply voltage. Circuit

layout and construction are also critical because stray capacitances

and lead inductances can form resonant circuits and are a

potential source of circuit peaking, oscillation, or both.

AD603 Datasheet(PDF) 20 Page - Analog Devices

AD603 Datasheet(HTML) 20 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

Datasheet Download

Link URL