Analog Devices AD600JR-REEL, AD600JR, AD600JN, AD600AR-REEL7, AD600AR-REEL Datasheet

a		Dual, Low Noise, Wideband
		Variable Gain Amplifiers
		AD600/AD602*

FEATURES

Two Channels with Independent Gain Control “Linear in dB” Gain Response

Two Gain Ranges: AD600: 0 dB to 40 dB

AD602: –10 dB to +30 dB Accurate Absolute Gain: 0.3 dB

Low Input Noise: 1.4 nV/√Hz

Low Distortion: –60 dBc THD at 1 V Output

High Bandwidth: DC to 35 MHz (–3 dB) Stable Group Delay: 2 ns

Low Power: 125 mW (Max) per Amplifier Signal Gating Function for Each Amplifier Drives High-Speed A/D Converters

MIL-STD-883-Compliant and DESC Versions Available

APPLICATIONS

Ultrasound and Sonar Time-Gain Control

High-Performance Audio and RF AGC Systems

Signal Measurement

PRODUCT DESCRIPTION

The AD600 and AD602 dual channel, low noise variable gain amplifiers are optimized for use in ultrasound imaging systems, but are applicable to any application requiring very precise gain, low noise and distortion, and wide bandwidth. Each independent channel provides a gain of 0 dB to +40 dB in the AD600 and –10 dB to +30 dB in the AD602. The lower gain of the AD602 results in an improved signal-to-noise ratio at the output. However, both products have the same 1.4 nV/√Hz input noise spectral density. The decibel gain is directly proportional to the control voltage, is accurately calibrated, and is supplyand temperature-stable.

To achieve the difficult performance objectives, a proprietary circuit form—the X-AMP®—has been developed. Each channel of the X-AMP comprises a variable attenuator of 0 dB to –42.14 dB followed by a high speed fixed gain amplifier. In this way, the amplifier never has to cope with large inputs, and can benefit from the use of negative feedback to precisely define the gain and dynamics. The attenuator is realized as a seven-stage R-2R ladder network having an input resistance of 100 Ω, lasertrimmed to ±2%. The attenuation between tap points is 6.02 dB; the gain-control circuit provides continuous interpolation between these taps. The resulting control function is linear in dB.

X-AMP is a registered trademark of Analog Devices, Inc. *Patented.

FUNCTIONAL BLOCK DIAGRAM

							GAT1
	SCALING			PRECISION PASSIVE			GATING
				INPUT ATTENUATOR
	REFERENCE						INTERFACE
	C1HI
	VG						A1OP
	C1LO
	GAIN CONTROL						A1CM
	INTERFACE						RF2
							2.24k (AD600)
	0dB	–12.04dB		–22.08dB	–36.12dB		694 (AD602)
	–6.02dB		–18.06dB	–30.1dB		–42.14dB	RF1
	A1HI						20
							FIXED-GAIN
	A1LO						AMPLIFIER
	500	R – 2R LADDER NETWORK				62.5	41.07dB(AD600)
							31.07(AD602)

The gain-control interfaces are fully differential, providing an input resistance of ~15 MΩ and a scale factor of 32 dB/V (that is, 31.25 mV/dB) defined by an internal voltage reference. The response time of this interface is less than 1 µs. Each channel also has an independent gating facility that optionally blocks signal transmission and sets the dc output level to within a few millivolts of the output ground. The gating control input is TTL and CMOS compatible.

The maximum gain of the AD600 is 41.07 dB, and that of the AD602 is 31.07 dB; the –3 dB bandwidth of both models is nominally 35 MHz, essentially independent of the gain. The signal-to-noise ratio (SNR) for a 1 V rms output and a 1 MHz noise bandwidth is typically 76 dB for the AD600 and 86 dB for the AD602. The amplitude response is flat within ±0.5 dB from 100 kHz to 10 MHz; over this frequency range the group delay varies by less than ±2 ns at all gain settings.

Each amplifier channel can drive 100 Ω load impedances with low distortion. For example, the peak specified output is ±2.5 V minimum into a 500 Ω load, or ± 1 V into a 100 Ω load. For a 200 Ω load in shunt with 5 pF, the total harmonic distortion for a ±1 V sinusoidal output at 10 MHz is typically –60 dBc.

The AD600J and AD602J are specified for operation from 0°C to 70°C, and are available in both 16-lead plastic DIP (N) and 16-lead SOIC (R). The AD600A and AD602A are specified for operation from –40°C to +85°C and are available in both 16-lead cerdip (Q) and 16-lead SOIC (R).

The AD600S and AD602S are specified for operation from –55°C to +125°C and are available in a 16-lead cerdip (Q) package and are MIL-STD-883 compliant. The AD600S and AD602S are also available under DESC SMD 5962-94572.

REV. B

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: [phone]	World Wide Web Site: http://www.analog.com
Fax: [phone]	© Analog Devices, Inc., 2001

AD600/AD602–SPECIFICATIONS (Each amplifier section, at TA = 25 C, VS = 5 V, –625 mV ≤ VG ≤ +625 mV, RL = 500 , and CL = 5 pF, unless otherwise noted. Specifications for AD600 and AD602 are identical unless otherwise noted.)

		AD600J/AD602J			AD600A/AD602A
Parameter	Conditions	Min	Typ	Max	Min	Typ	Max	Unit
INPUT CHARACTERISTICS								Ω
Input Resistance	Pins 2 to 3; Pins 6 to 7	98	100	102	95	100	105
Input Capacitance			2			2		pF
Input Noise Spectral Density1	RS = 50 Ω, Maximum Gain		1.4			1.4		nV/√Hz
Noise Figure			5.3			5.3		dB
	RS = 200 Ω, Maximum Gain		2			2		dB
Common-Mode Rejection Ratio	f = 100 kHz		30			30		dB
OUTPUT CHARACTERISTICS
–3 dB Bandwidth	VOUT = 100 mV rms		35			35		MHz
Slew Rate			275			275		V/µs
Peak Output2	RL ≥ 500 Ω	±2.5	±3		±2.5	±3		V
Output Impedance	f ≤ 10 MHz		2			2		Ω
Output Short-Circuit Current			50			50		mA
Group Delay Change vs. Gain	f = 3 MHz; Full Gain Range		±2			±2		ns
Group Delay Change vs. Frequency	VG = 0 V, f = 1 MHz to 10 MHz		±2			±2		ns
Total Harmonic Distortion	RL= 200 Ω, VOUT = ±1 V Peak, Rpd = 1 kΩ		–60			–60		dBc
ACCURACY
AD600
Gain Error	0 dB to 3 dB Gain	0	+0.5	+1	–0.5	+0.5	+1.5	dB
	3 dB to 37 dB Gain	–0.5	±0.2	+0.5	–1.0	±0.2	+1.0	dB
Maximum Output Offset Voltage3	37 dB to 40 dB Gain	–1	–0.5	0	–1.5	–0.5	+0.5	dB
	VG = –625 mV to +625 mV		10	50		10	65	mV
Output Offset Variation	VG = –625 mV to +625 mV		10	50		10	65	mV
AD602
Gain Error	–10 dB to –7 dB Gain	0	+0.5	+1	–0.5	+0.5	+1.5	dB
	–7 dB to +27 dB Gain	–0.5	±0.2	+0.5	–1.0	±0.2	+1.0	dB
Maximum Output Offset Voltage3	27 dB to 30 dB Gain	–1	–0.5	0	–1.5	–0.5	+0.5	dB
	VG = –625 mV to +625 mV		5	30		10	45	mV
Output Offset Variation	VG = –625 mV to +625 mV		5	30		10	45	mV
GAIN CONTROL INTERFACE
Gain Scaling Factor	3 dB to 37 dB (AD600); –7 dB to +27 dB (AD602)	31.7	32	32.3	30.5	32	33.5	dB/V
Common-Mode Range		–0.75		+2.5	–0.75		+2.5	V
Input Bias Current			0.35	1		0.35	1	µA
Input Offset Current			10	50		10	50	nA
Differential Input Resistance	Pins 1 to 16; Pins 8 to 9		15			15		MΩ
Response Rate	Full 40 dB Gain Change		40			40		dB/µs
SIGNAL GATING INTERFACE
Logic Input “LO” (Output ON)				0.8			0.8	V
Logic Input “HI” (Output OFF)		2.4			2.4			V
Response Time	ON to OFF, OFF to ON		0.3			0.3		µs
Input Resistance	Pins 4 to 3; Pins 5 to 6		30			30		kΩ
Output Gated OFF			±10			±10
Output Offset Voltage				100			400	mV
Output Noise Spectral Density			65			65		nV/√Hz
Signal Feedthrough @ 1 MHz
AD600			–80			–80		dB
AD602			–70			–70		dB
POWER SUPPLY		±4.75		±5.25	±4.75		±5.25
Specified Operating Range								V
Quiescent Current			11	12.5		22	28	mA

NOTES

1Typical open or short-circuited input; noise is lower when system is set to maximum gain and input is short-circuited. This figure includes the effects of both voltage and current noise sources.

2Using resistive loads of 500 Ω or greater, or with the addition of a 1 kΩ pull-down resistor when driving lower loads.

3The dc gain of the main amplifier in the AD600 is X113; thus an input offset of only 100 µV becomes an 11.3 mV output offset. In the AD602, the amplifier’s gain is X35.7; thus, an input offset of 100 µV becomes a 3.57 mV output offset.

Specifications shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. All min and max specifications guaranteed, although only those shown in boldface are tested on all production units.

Specifications subject to change without notice.

–2–

REV. B

AD600/AD602

ABSOLUTE MAXIMUM RATINGS1
Supply Voltage ±VS . . . . . . . . . . . . . . . . . . . .	. . . . . . . ±7.5 V
Input Voltages	±VS
Pins 1, 8, 9, 16 . . . . . . . . . . . . . . . . . . . . . .
Pins 2, 3, 6, 7 . . . . . . . . . . . . . . . . . . . . . .	±2 V Continuous
. . . . . . . . . . . . . . . . . . . . . . .	. . ±VS for 10 ms
Pins 4, 5 . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . . . . . . . ±VS
Internal Power Dissipation2 . . . . . . . . . . . . . .	. . . . . . 600 mW
Operating Temperature Range (J) . . . . . . . . .	. . . 0°C to 70°C
Operating Temperature Range (A) . . . . . . . .	–40°C to +85°C
Operating Temperature Range (S) . . . . . . .	–55°C to +125°C
Storage Temperature Range . . . . . . . . . . . .	–65°C to +150°C
Lead Temperature Range (Soldering 60 sec) .	. . . . . . . . 300°C

NOTES

1Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

2Thermal Characteristics:

16-Lead Plastic Package: θJA = 85°C/W

16-Lead SOIC Package: θJA = 100°C/W

16-Lead Cerdip Package: θJA = 120°C/W

ORDERING GUIDE

	Gain	Temperature	Package
Model	Range	Range	Option1
AD600AQ	0 dB to 40 dB	–40°C to +85°C	Q-16
AD600AR	0 dB to 40 dB	–40°C to +85°C	R-16
AD600AR-REEL	0 dB to 40 dB	–40°C to +85°C	13" Reel
AD600AR-REEL7	0 dB to 40 dB	–40°C to +85°C	7" Reel
AD600JN	0 dB to 40 dB	0°C to 70°C	N-16
AD600JR	0 dB to 40 dB	0°C to 70°C	R-16
AD600JR-REEL	0 dB to 40 dB	0°C to 70°C	13" Reel
AD600JR-REEL7	0 dB to 40 dB	0°C to 70°C	7" Reel
AD600SQ/883B2	0 dB to 40 dB	–55°C to +125°C	Q-16
AD602AQ	–10 dB to +30 dB	–40°C to +85°C	Q-16
AD602AR	–10 dB to +30 dB	–40°C to +85°C	R-16
AD602AR-REEL	–10 dB to +30 dB	–40°C to +85°C	13" Reel
AD602AR-REEL7	–10 dB to +30 dB	–40°C to +85°C	7" Reel
AD602JN	–10 dB to +30 dB	0°C to 70°C	N-16
AD602JR	–10 dB to +30 dB	0°C to 70°C	R-16
AD602JR-REEL	–10 dB to +30 dB	0°C to 70°C	13" Reel
AD602JR-REEL7	–10 dB to +30 dB	0°C to 70°C	7" Reel
AD602SQ/883B3	–10 dB to +30 dB	–55°C to +150°C	Q-16

NOTES

1N = Plastic DIP; Q = Cerdip; R = Small Outline IC (SOIC).

2Refer to AD600/AD602 Military data sheet. Also available as 5962-9457201MEA. 3Refer to AD600/AD602 Military data sheet. Also available as 5962-9457202MEA.

PIN FUNCTION DESCRIPTIONS

Pin	Mnemonic		Description
1	C1LO		CH1 Gain-Control Input “LO” (Positive
			Voltage Reduces CH1 Gain).
2	A1HI		CH1 Signal Input “HI” (Positive Voltage
			Increases CH1 Output).
3	A1LO		CH1 Signal Input “LO” (Usually Taken to
			CH1 Input Ground)
4	GAT1		CH1 Gating Input (A Logic “HI” Shuts Off
			CH1 Signal Path).
5	GAT2		CH2 Gating Input (A Logic “HI” Shuts Off
			CH2 Signal Path).
6	A2LO		CH2 Signal Input “LO” (Usually Taken to
			CH2 Input Ground).
7	A2HI		CH2 Signal Input “HI” (Positive Voltage
			Increases CH2 Output).
8	C2LO		CH2 Gain-Control Input “LO” (Positive
			Voltage Reduces CH2 Gain).
9	C2HI		CH2 Gain-Control Input “HI” (Positive
			Voltage Increases CH2 Gain).
10	A2CM		CH2 Common (Usually Taken to CH2
			Output Ground).
11	A2OP		CH2 Output.
12	VNEG		Negative Supply for Both Amplifiers.
13	VPOS		Positive Supply for Both Amplifiers.
14	A1OP		CH1 Output.
15	A1CM		CH1 Common (Usually Taken to CH1
			Output Ground).
16	C1HI		CH1 Gain-Control Input “HI” (Positive
			Voltage Increases CH1 Gain).
		CONNECTION DIAGRAM
	16-Lead Plastic DIP (N) Package
	16-Lead Plastic SOIC (R) Package
		16-Lead Cerdip (Q) Package

C1LO	1			16	C1HI
A1HI	2	+	A1	15	A1CM
A1LO	3	–		14	A1OP
GAT1	4		REF	13	VPOS
GAT2	5			12	VNEG
A2LO	6	–	A2	11	A2OP
A2HI	7	+		10	A2CM
C2LO	8	AD600 / AD602		9	C2HI

CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD600/AD602 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.

WARNING!

ESD SENSITIVE DEVICE

REV. B

–3–

AD600/AD602–Typical Performance Characteristics

	0.45
	0.35
dB	0.25
	0.15
–	–0.05
ERROR
GAIN	0.05
	–0.15
	–0.25
	–0.35
	–0.45
	–0.7 –0.5 –0.3 –0.1 0.1 0.3 0.5 0.7
		GAIN CONTROL VOLTAGE – Volts

TPC 1. Gain Error vs. Gain Control

Voltage
	10.0
	9.8
	9.6
ns	9.4
–	9.2
DELAY
	9.0
GROUP	8.8
	8.6
	8.4
	8.2
	8.0
	–0.7	–0.5	–0.3	–0.1	0.1	0.3	0.5	0.7

GAIN CONTROL VOLTAGE – Volts

TPC 4. AD600 and AD602 Typical Group Delay vs. VC

	102
	101	GAIN = 40dB
–	100
	99
IMPEDANCE
	98		GAIN = 20dB
	97		GAIN = 0dB
	96
INPUT
	95
	94
	93
	92
	100k	1M	10M	100M
		FREQUENCY – Hz

TPC 7. Input Impedance vs. Frequency

20dB
17dB
0
–45
–90
100k	1M	10M	100M

FREQUENCY – Hz

TPC 2. AD600 Frequency and Phase Response vs. Gain

VG = 0V 10dB/DIV CENTER FREQ 1MHz 10kHz/DIV

TPC 5. Third Order Intermodula-

tion Distortion, VOUT = 2 V p-p,
RL = 500 Ω
	6
– mV	5
	4				AD600
VOLTAGE	3
	2			AD602
OFFSET	1
	0
OUTPUT	–1
	–2
	–3
	–4
	–0.7	–0.5	–0.3	–0.1	0.1	0.3	0.5	0.7

GAIN CONTROL VOLTAGE – Volts

TPC 8. Output Offset vs. Gain Control Voltage (Control Channel Feedthrough)

10dB
7dB
0
–45
–90
100k	1M	10M	100M

FREQUENCY – Hz

TPC 3. AD602 Frequency and Phase Response vs. Gain

Volts	–1.0
	–1.2
–	–1.4
LIMIT
	–1.6
VOLTAGE	–1.8
	–2.0
	–2.2
OUTPUT	–2.4
	–2.6
	–2.8
NEGATIVE
	–3.0
	–3.2
	–3.4
		50	100	200	500	1000	2000
	0

LOAD RESISTANCE –

TPC 6. Typical Output Voltage vs. Load Resistance (Negative Output Swing Limits First)

	1V VOUT	1µs
	100
OUTPUT	90
INPUT	10
	0%

1V VC

TPC 9. Gain Control Channel Response Time. Top: Output Voltage, 2 V max, Bottom: Gain Control Voltage VC = ±625 mV

–4–

REV. B

AD600/AD602

		50mV
OUTPUT	100
	90
INPUT	10
	0%
	5V	100ns

TPC 10. Gating Feedthrough to Output, Gating Off to On

50mV

OUTPUT	100
	90
INPUT	10
	0%

5V

100ns

TPC 11. Gating Feedthrough to Output, Gating On to Off

	1V
	100
OUTPUT	90
INPUT	10
	0%
	100mV	500ns

TPC 12. Transient Response,

Medium and High Gain

OUTPUT		500mV	OUTPUT		1V
	100			100
	90			90
INPUT	10		INPUT	10
	0%			0%
	1V	200ns		200mV	500ns

TPC 13. Input Stage Overload	TPC 14. Output Stage Overload
Recovery Time	Recovery Time

			500mV
		100
	OUTPUT	90
	INPUT	10
		0%
		1V	500ns

TPC 15. Transient Response

Minimum Gain

	10
	5	AD600: G = 20dB
		AD602: G = 10dB
	0	BOTH: VCM = 100mV RMS
		VS =	5V
	–5	RL = 500
dB	–10	TA = 25 C
–	–15
CMRR
	–20		AD600
	–25
	–30			AD602
	–35
	–40
	1k	10k	100k	1M	10M	100M
		FREQUENCY – Hz

TPC 16. CMRR vs. Frequency

	20
	10
	0
	–10	AD600
– dB	–20
	–30
PSRR			AD602
	–40
	–50		AD600: G = 40dB
			AD602: G = 30dB
	–60		BOTH: RL = 500
	–70		VIN = 0V
			RS = 50
	–80
	100k	1M	10M	100M
		FREQUENCY – Hz

TPC 17. PSRR vs. Frequency

	10
	0	AD600: CH1 G = 40dB, VIN = 0
			CH2 G = 20dB, VIN = 100mV
	–10	AD602: CH1 G = 30dB, VIN = 0
			CH2 G = 0dB, VIN = 316mV
– dB	–20	BOTH: VOUT = 1V RMS1, RS = 50 ,
			RL = 500	CH1 V	}
	–30	CROSSTALK = 20log { CH2 VOUTIN
CROSSTALK	–40
	–50			AD600
	–60
	–70			AD602
	–80
	–90
	100k		1M	10M	100M
			FREQUENCY – Hz

TPC 18. Crosstalk Between A1 and A2 vs. Frequency

REV. B

–5–

AD600/AD602

THEORY OF OPERATION

The AD600 and AD602 have the same general design and features. They comprise two fixed gain amplifiers, each preceded by a voltage-controlled attenuator of 0 dB to 42.14 dB with independent control interfaces, each having a scaling factor of 32 dB per volt. The gain of each amplifier in the AD600 is laser trimmed to 41.07 dB (X113), thus providing a control range of –1.07 dB to 41.07 dB (0 dB to 40 dB with overlap), while the AD602 amplifiers have a gain of 31.07 dB (X35.8) and provide an overall gain of –11.07 dB to 31.07 dB (–10 dB to 30 dB with overlap).

The advantage of this topology is that the amplifier can use negative feedback to increase the accuracy of its gain; also, since the amplifier never has to handle large signals at its input, the distortion can be very low. A further feature of this approach is that the small-signal gain and phase response, and thus the pulse response, are essentially independent of gain.

The following discussion describes the AD600. Figure 1 is a simplified schematic of one channel. The input attenuator is a seven-section R-2R ladder network, using untrimmed resistors of nominally R = 62.5 Ω, which results in a characteristic resistance of 125 Ω ± 20%. A shunt resistor is included at the input and laser trimmed to establish a more exact input resistance of 100 Ω ± 2%, which ensures accurate operation (gain and HP corner frequency) when used in conjunction with external resistors or capacitors.

							GAT1
	SCALING			PRECISION PASSIVE			GATING
				INPUT ATTENUATOR
	REFERENCE						INTERFACE
	C1HI
	VG						A1OP
	C1LO
	GAIN CONTROL						A1CM
	INTERFACE						RF2
							2.24k (AD600)
	0dB	–12.04dB		–22.08dB	–36.12dB		694 (AD602)
	–6.02dB		–18.06dB	–30.1dB		–42.14dB	RF1
	A1HI						20
							FIXED-GAIN
	A1LO						AMPLIFIER
	500	R – 2R LADDER NETWORK				62.5	41.07dB(AD600)
							31.07(AD602)

Figure 1. Simplified Block Diagram of Single Channel of the AD600 and AD602

The nominal maximum signal at input A1HI is 1 V rms (±1.4 V peak) when using the recommended ± 5 V supplies, although operation to ±2 V peak is permissible with some increase in HF distortion and feedthrough. Each attenuator is provided with a separate signal “LO” connection, for use in rejecting commonmode, the voltage between input and output grounds. Circuitry is included to provide rejection of up to ±100 mV.

The signal applied at the input of the ladder network is attenuated by 6.02 dB by each section; thus, the attenuation to each of the taps is progressively 0, 6.02, 12.04, 18.06, 24.08, 30.1, 36.12 and 42.14 dB. A unique circuit technique is employed to interpolate between these tap points, indicated by the “slider” in Figure 1, providing continuous attenuation from 0 dB to 42.14 dB.

It will help, in understanding the AD600, to think in terms of a mechanical means for moving this slider from left to right; in fact, it is voltage controlled. The details of the control interface are discussed later. Note that the gain is at all times exactly determined, and a linear decibel relationship is automatically guaranteed between the gain and the control parameter which determines the position of the slider. In practice, the gain deviates from the ideal law, by about ±0.2 dB peak (see, for example, Figure 6).

Note that the signal inputs are not fully differential: A1LO and A1CM (for CH1) and A2LO and A2CM (for CH2) provide separate access to the input and output grounds. This recognizes the practical fact that even when using a ground plane, small differences will arise in the voltages at these nodes. It is important that A1LO and A2LO be connected directly to the input ground(s); significant impedance in these connections will reduce the gain accuracy. A1CM and A2CM should be connected to the load ground(s).

Noise Performance

An important reason for using this approach is the superior noise performance that can be achieved. The nominal resistance seen at the inner tap points of the attenuator is 41.7 Ω (one third

of 125 Ω), which exhibits a Johnson noise spectral density (NSD) of 0.84 nV/√Hz (that is, √4kTR) at 27°C, which is a large fraction

of the total input noise. The first stage of the amplifier contributes a further 1.12 nV/√Hz, for a total input noise of 1.4 nV/√Hz.

The noise at the 0 dB tap depends on whether the input is short-circuited or open-circuited: when shorted, the minimum NSD of 1.12 nV/√Hz is achieved; when open, the resistance of 100 Ω at the first tap generates 1.29 nV/√Hz, so the noise increases to a total of 1.71 nV/√Hz. (This last calculation would be important if the AD600 were preceded, for example, by a 900 Ω resistor to allow operation from inputs up to ±10 V rms. However, in most cases the low impedance of the source will limit the maximum noise resistance.)

It will be apparent from the foregoing that it is essential to use a low resistance in the design of the ladder network to achieve low noise. In some applications this may be inconvenient, requiring the use of an external buffer or preamplifier. However, very few amplifiers combine the needed low noise with low distortion at maximum input levels, and the power consumption needed to achieve this performance is fundamentally required to be quite high (due to the need to maintain very low resistance values while also coping with large inputs). On the other hand, there is little value in providing a buffer with high input impedance, since the usual reason for this—the minimization of loading of a high resistance source—is not compatible with low noise.

Apart from the small variations just discussed, the signal-to- noise (S/N) ratio at the output is essentially independent of the attenuator setting, since the maximum undistorted output is 1 V rms and the NSD at the output of the AD600 is fixed at 113 times 1.4 nV/√Hz, or 158 nV/√Hz. Thus, in a 1 MHz bandwidth, the output S/N ratio would be 76 dB. The input NSD of the AD600 and AD602 are the same, but because of the 10 dB lower gain in the AD602’s fixed amplifier, its output S/N ratio is 10 dB better, or 86 dB in a 1 MHz bandwidth.

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REV. B