Analog Devices manual AD600/AD602, U1 AD600, U2 AD600, AD636

AD600/AD602

INPUT

1V RMS

MAX

(SINE WAVE)

C1LO

A1HI

A1LO

GAT1

GAT2

A2LO

A2HI C2LO

R13

3.01kΩ

1

16

C1HI

C1LO

1

16

C1HI

R3

2

15

A1CM

R2

A1HI

2

15

A1CM

C1

200Ω

487Ω

A1

A1OP

∝

U3A

A1LO

A1

A1OP

3

14

0.1 F

3

14

4

13

VPOS

+5V DEC

1/4

GAT1

4

13

VPOS

+5V DEC

REF

VNEG

R1

AD713

GAT2

REF

VNEG

5

12

–5V DEC 133kΩ

5

C2

12

–5V DEC

A2OP

R5

A2LO

6

11

0.1∝F

6

11

A2OP

1.58kΩ

A2

A2

A2CM

A2HI

A2CM

7

10

7

10

U3B

C2HI

R4

C3

C2LO

C2HI

8

9

8

9

133kΩ

220pF

1/4

U1 AD600

U2 AD600

AD713

+5V

–2dB

+2dB

FB

–62.5mV

0dB

+62.5mV

0.1∝F

–5V

+5V

+5V DEC

R6

R7

R8

R9

10kΩ

127Ω

127Ω

10kΩ

–5V DEC

FB

0.1∝F

C5

–5V

22∝F

+5V DEC

POWER SUPPLY

DECOUPLING NETWORK

1

VINP

VPOS

14

+5V DEC

NC

2

U4

13

NC

AD636

R11

R15

–5V DEC

3

VNEG

12

NC

46.4kΩ

19.6kΩ

C6

4

CAVG

11

NC

R10

4.7∝F

3.16kΩ

R16

NC

5

VLOG

COMM

10

Q1

6.65kΩ

2N3906

R14

NC

6

BFOP

LDLO

9

301kΩ

7

8

U3C

VLOG

BFIN

VRMS

1/4

R12

+316.2mV

AD713

11.3kΩ

NC = NO CONNECT

C4

2∝F VOUT

Figure 25. RMS Responding AGC Circuit with 100 dB Dynamic Range

100 dB to 120 dB RMS Responding Constant Bandwidth AGC Systems with High Accuracy dB Outputs

The next two applications double as both AGC amplifiers and measurement systems. In both, precise gain offsets are used to achieve either (1) a very high gain linearity of ± 0.1 dB over the full 100 dB range, or (2) the optimal signal-to-noise ratio at any gain.

A 100 dB RMS/AGC System with Minimal Gain Error (Parallel Gain with Offset)

Figure 25 shows an rms-responding AGC circuit, which can equally well be used as an accurate measurement system. It accepts inputs of 10 μV to 1 V rms (–100 dBV to 0 dBV) with generous overrange. Figure 26 shows the logarithmic output, VLOG, which is accurately scaled 1 V per decade, that is,

50 mV/dB, with an intercept (VLOG = 0) at 3.16 mV rms

(–50 dBV). Gain offsets of ± 2 dB have been introduced between the amplifiers, provided by the ± 62.5 mV introduced by R6–R9. These offsets cancel a small gain ripple which arises in the X-AMP from its finite interpolation error, which has a period of 18 dB in the individual VCA sections. The gain ripple of all three amplifier sections without this offset (in which case the gain errors simply add) is shown in Figure 27; it is still a

remarkably low ± 0.25 dB over the 108 dB range from 6 μV to

1.5V rms. However, with the gain offsets connected, the gain linearity remains under ± 0.1 dB over the specified 100 dB range (Figure 28).

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

INPUT SIGNAL – V RMS

Figure 26. VLOG Plotted vs. VIN for Figure 25‘s Circuit Showing 120 dB AGC Range

–14– REV. A