Analog Devices manual AD600/AD602

	2.0
	1.5
	1.0
dB	0.5
–
	–0.2
ERROR
	0.2
GAIN	0
	–0.5
	–1.0
	–1.5
	–2.0
	1∝V	10∝V 100∝V 1mV 10mV 100mV	1V	10V

INPUT SIGNAL – V RMS

Figure 31. The Error Ripple Due to the Individual Gain Functions

	400
	350
– mV
ERRORGAIN	300
	250
	200
	1∝V	10∝V 100∝V 1mV 10mV 100mV	1V	10V

INPUT SIGNAL – V RMS

Figure 32. VAGC Remains Nose to Its Setpoint of 316 mV RMS Over the Full 120 dB Range

	90
	80
	70
– dB	60
	50
RATIO
	40
S/N	30
	20
	10
	0
	–833.2	–625.0	–416.6	–208.3	0	208.3	416.6	625.0	833.2

CONTROL VOLTAGE, VC (10.417mV/dB) – mV

Figure 33. S/N Ratio vs. Control Voltage for Parallel Gain Control (Figure 25)

AD600/AD602

In contrast, the S/N ratio for the sequential mode is shown in Figure 34. U1A always acts as a fixed noise source; varying its gain has no influence on the output noise. (This is a feature of the X-AMP technique.) Thus, for the first 40 dB of control range (actually slightly more, as explained below), when only this VCA section has its gain varied, the S/N ratio remains con- stant. During this time, the gains of U1B and U2A are at their minimum value of –1.07 dB.

	90
	80
	70
– dB	60
	50
RATIO
	40
S/N	30
	20
	10
	0
	–1.183	–0.558	0.067	0.692	1.317	1.942	2.567	3.192	3.817

CONTROL VOLTAGE, VC (31.25mV/dB) – Volts

Figure 34. S/N Ratio vs. Control Voltage for Sequential Gain Control (Figure 29)

For the next 40 dB of control range, the gain of U1A remains fixed at its maximum value of 41.07 dB and only the gain of U1B is varied, while that of U2A remains at its minimum value of –1.07 dB. In this interval, the fixed output noise of U1A is amplified by the increasing gain of U1B and the S/N ratio pro- gressively decreases.

Once U1B reaches its maximum gain of 41.07 dB, its output also becomes a gain independent noise source; this noise is pre- sented to U2A. As the control voltage is further increased, the gains of both U1A and U1B remain fixed at their maximum value of 41.07 dB, and the S/N ratio continues to decrease. Fig- ure 34 clearly shows this, because the maximum S/N ratio of 90 dB is extended for the first 40 dB of input signal before it starts to roll off.

This arrangement of staggered gains can be easily implemented because, when the control inputs of the AD600 are overdriven, the gain limits to its maximum or minimum values without side effects. This eliminates the need for awkward nonlinear shaping circuits that have previously been used to break up the gain range of multistage AGC amplifiers. It is the precise values of the AD600’s maximum and minimum gain (not 0 dB and

40 dB but –1.07 dB and 41.07 dB) that explain the rather odd values of the offset values that are used.

The optimization of the output S/N ratio is of obvious value in AGC systems. However, in applications where these circuit are considered for their wide range logarithmic measurements capa- bilities, the inevitable degradation of the S/N ratio at high gains need not seriously impair their utility. In fact, the bandwidth of the circuit shown in Figure 25 was specifically chosen so as to improve measurement accuracy by altering the shape of the log error curve (Figure 31) at low signal levels.

REV. A –17–