AD600ARZ-R7 datasheet(23/28 Pages) AD

23 / 28 page

AD600/AD602

Rev. E | Page 23 of 28

2.0

–2.0

0.5

1.0

1.5

–1.5

–1.0

–0.5

–0.1

0.1

1µ

10µ

100m

10m

100µ

INPUT SIGNAL (V rms)

Figure 49. Gain Error for Figure 41 Without the 2 dB Offset Modification

2.0

–2.0

0.5

1.0

1.5

–1.5

–1.0

–0.5

–0.1

0.1

1µ

10µ

100m

10m

100µ

INPUT SIGNAL (V rms)

Figure 50. Adding the 2 dB Offsets Improves the Linearization

The maximum gain of this circuit is 120 dB. If no filtering was

used, the noise spectral density of the AD600 (1.4 nV/√Hz)

would amount to an input noise of 8.28 μV rms in the full

bandwidth (35 MHz). At a gain of one million, the output noise

would dominate. Consequently, some reduction of bandwidth is

mandatory, and in the circuit of Figure 47, it is due mostly to a

single-pole, low-pass filter R5/C3, which provides a −3 dB

frequency of 458 kHz, which reduces the worst-case output

noise (at VAGC) to about 100 mV rms at a gain of 100 dB. Of

course, the bandwidth (and therefore the output noise) could be

further reduced, for example, in audio applications, merely by

increasing C3. The value chosen for this application is optimal

in minimizing the error in the VLOG output for small input signals.

The AD600 is dc-coupled, but even miniscule offset voltages at

the input would overload the output at high gains; thus, high-

pass filtering is also needed. To provide operation at low

frequencies, two simple 0s at about 12 Hz are provided by

R1/C1 and R4/C2; op amp sections U3A and U3B (AD713)

are used to provide impedance buffering, because the input

resistance of the AD600 is only 100 Ω. A further 0 at 12 Hz is

provided by C4 and the 6.7 kΩ input resistance of the AD636

rms converter.

The rms value of VLOG is generated at Pin 8 of the AD636; the

averaging time for this process is determined by C5, and the

value shown results in less than 1% rms error at 20 Hz. The

slowly varying V rms is compared with a fixed reference of

316 mV, derived from the positive supply by R10/R11. Any

difference between these two voltages is integrated in C6, in

conjunction with Op Amp U3C, the output of which is VLOG. A

fraction of this voltage, determined by R12 and R13, is returned

to the gain control inputs of all AD600 sections. An increase in

VLOG lowers the gain because this voltage is connected to the

inverting polarity control inputs.

In this case, the gains of all three VCA sections are being varied

simultaneously, so the scaling is not 32 dB/V but 96 dB/V or

10.42 mV/dB. The fraction of VLOG required to set its scaling to

50 mV/dB is therefore 10.42/50 or 0.208. The resulting full-

scale range of VLOG is nominally ±2.5 V. This scaling allows the

circuit to operate from ±5 V supplies.

Optionally, the scaling can be altered to 100 mV/dB, which

would be more easily interpreted when VLOG is displayed on a

DVM by increasing R12 to 25.5 kΩ. The full-scale output of

±5 V then requires the use of supply voltages of at least ±7.5 V.

A simple attenuator of 16.6 ± 1.25 dB is formed by R2/R3

and the 100 Ω input resistance of the AD600. This allows the

reference level of the decibel output to be precisely set to 0 for

an input of 3.16 mV rms, and thus center the 100 dB range

between 10 μV and 1 V. In many applications, R2/R3 can be

replaced by a fixed resistor of 590 Ω. For example, in AGC

applications, neither the slope nor the intercept of the

logarithmic output is important.

A few additional components (R14 to R16 and Q1) improve the

accuracy of VLOG at the top end of the signal range (that is, for

small gains). The gain starts rolling off when the input to the

first amplifier, U1A, reaches 0 dB. To compensate for this

nonlinearity, Q1 turns on at VLOG ~ 1.5 V and increases the

feedback to the control inputs of the AD600s, thereby needing a

smaller voltage at VLOG to maintain the input to the AD636 to

the setpoint of 316 mV rms.

A 120 dB RMS/AGC SYSTEM WITH OPTIMAL SNR

(SEQUENTIAL GAIN)

In the last case, all gains were adjusted simultaneously, resulting

in an output SNR that is always less than optimal. The use of

sequential gain control results in a major improvement in SNR,

with only a slight penalty in the accuracy of VLOG, and no

penalty in the stabilization accuracy of VAGC. The idea is to

increase the gain of the earlier stages first (as the signal level

decreases) and maintain the highest SNR throughout the

amplifier chain. This can be easily achieved with the AD600

because its gain is accurate even when the control input is

overdriven. That is, each gain control window of 1.25 V is

used fully before moving to the next amplifier to the right.

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