Analog Devices manual AD600/AD602, Rev. A

(This system can, of course, be used as an AGC amplifier, in which the rms value of the input is leveled.) Figure 21 shows the “decibel” output voltage. More revealing is Figure 22, which shows that the deviation from the ideal output predicted by Equation 1 over the input range 80 μV to 500 mV rms is within

	450
	425
	400
	375
mV–	350
	325
OUT	300
	275
V
	250
	225
	200
	175
	150
	10∝V	100∝V	1mV	10mV	100mV	1V	10V

INPUT SIGNAL –V RMS

Figure 20. The RMS Output of A2 Is Held Close to the “Setpoint” 316 mV for an Input Range of Over 80 dB

	5
	4
	3
Volts–	2
	0
	1
OUT	–1
V
	–2
	–3
	–4
	–5
	10∝V	100∝V	1mV	10mV	100mV	1V	10V

INPUT SIGNAL – V RMS

Figure 21. The dB Output of Figure 19’s Circuit Is Linear Over an 80 dB Range

	2.5
	2.0
	1.5
– dB	1.0
	0.5
ERROR
	0
OUTPUT	–1.0
	–0.5
	–1.5
	–2.0
	–2.5
	10∝V	100∝V	1mV	10mV	100mV	1V	10V

INPUT SIGNAL – V RMS

Figure 22. Data from Figure 20 Presented as the Deviation from the Ideal Output Given in Equation 4

AD600/AD602

±0.5 dB, and within ± 1 dB for the 80 dB range from 80 μV to 800 mV. By suitable choice of the input attenuator R1 + R2, this could be centered to cover any range from 25 mV to 250 mV to, say, 1 mV to 10 V, with appropriate correction to the value

of VREF. (Note that VSCALE is not affected by the changes in the range.) The gain ripple of ± 0.2 dB seen in this curve is the re- sult of the finite interpolation error of the X-AMP. Note that it occurs with a periodicity of 12 dB—twice the separation be- tween the tap points (because of the two cascaded stages).

This ripple can be canceled whenever the X-AMP stages are cascaded by introducing a 3 dB offset between the two pairs of control voltages. A simple means to achieve this is shown in Figure 23: the voltages at C1HI and C2HI are “split” by

±46.875 mV, or ± 1.5 dB. Alternatively, either one of these pins can be individually offset by 3 dB and a 1.5 dB gain adjustment made at the input attenuator (R1 + R2).

	16	C1HI				1	VINP
	15	A1CM			NC	2
	14	A1OP		–6V		3	VNEG
				DEC
		VPOS					U2
U1	13		+6V DEC			4
							CAVG
AD600	12	VNEG	–6V DEC	C2	NC	5	AD636
				2∝F			VLOG
		A2OP
	11				NC	6	BFOP
	10	A2CM				7	BFIN
	9	C2HI
			–46.875mV		+46.875mV
	–6V					+6V
	DEC	10kΩ	78.7Ω	78.7Ω	10kΩ	DEC
					3dB OFFSET
NC = NO CONNECT					MODIFICATION

Figure 23. Reducing the Gain Error Ripple

The error curve shown in Figure 24 demonstrates that over the central portion of the range the output voltage can be main- tained very close to the ideal value. The penalty for this modifi- cation is the higher errors at the extremities of the range. The next two applications show how three amplifier sections can be cascaded to extend the nominal conversion range to 120 dB, with the inclusion of simple LP filters of the type shown in Fig- ure 15. Very low errors can then be maintained over a 100 dB range.

	2.5
	2.0
	1.5
– dB	1.0
	0.5
ERROR
	0
OUTPUT	–1.0
	–0.5
	–1.5
	–2.0
	–2.5
	10∝V	100∝V	1mV	10mV	100mV	1V	10V

INPUT SIGNAL – V RMS

Figure 24. Using the 3 dB Offset Network, the Ripple Is Reduced

REV. A –13–