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Dual, Ultralow Noise
Variable Gain Amplifier
AD604
FEATURES FUNCTIONAL BLOCK DIAGRAM
Ultralow input noise at maximum gain
PAOx –DSXx +DSXx VGNx
0.80 nV/√Hz, 3.0 pA/√Hz
2 independent linear-in-dB channels
GAIN CONTROL
VREF
AND SCALING
Absolute gain range per channel programmable
DIFFERENTIAL
ATTENUATOR
0 dB to 48 dB (preamplifier gain = 14 dB) through 6 dB to
54 dB (preamplifier gain = 20 dB)
R-1.5R
PAIx AFA OUTx
LADDER NETWORK
±1.0 dB gain accuracy
0dB TO –48.4dB
Bandwidth: 40 MHz
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AD604 TABLE OF CONTENTS Features .............................................................................................. 1 Applications Information .............................................................. 18 Applications ....................................................................................... 1 Ultralow Noise AGC Amplifier with 82 dB to 96 dB Gain Range .................................................................................. 19 Functional Block Dia
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AD604 SPECIFICATIONS Each amplifier channel at TA = 25°C, VS = ±5 V, RS = 50 Ω, RL = 500 Ω, CL = 5 pF, VREF = 2.50 V (scaling = 20 dB/V), 0 dB to 48 dB gain range (preamplifier gain = 14 dB), VOCM = 2.5 V, C1 and C2 = 0.1 μF (see Figure 37), unless otherwise noted. Table 1. Parameter Conditions Min Typ Max Unit INPUT CHARACTERISTICS Preamplifier Input Resistance 300 kΩ Input Capacitance 8.5 pF Input Bias Current −27 mA Peak Input Voltage Preampli
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AD604 Parameter Conditions Min Typ Max Unit ACCURACY Absolute Gain Error 0 dB to 3 dB 0.25 V < VGN < 0.400 V −1.2 +0.75 +3 dB 3 dB to 43 dB 0.400 V < VGN < 2.400 V −1.0 ±0.3 +1.0 dB 43 dB to 48 dB 2.400 V < VGN < 2.65 V −3.5 −1.25 +1.2 dB Gain Scaling Error 0.400 V < VGN < 2.400 V ±0.25 dB/V Output Offset Voltage VREF = 2.500 V, VOCM = 2.500 V −50 ±30 +50 mV Output Offset Variation VREF = 2.500 V, VOCM = 2.500 V 30 50 mV GAIN CONT
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AD604 ABSOLUTE MAXIMUM RATINGS Stresses above those listed under Absolute Maximum Ratings Table 2. may cause permanent damage to the device. This is a stress 1, 2 Parameter Rating rating only; functional operation of the device at these or any Supply Voltage ±V S other conditions above those indicated in the operational Pin 17 to Pin 20 (with Pin 16, Pin 22 = 0 V) ±6.5 V section of this specification is not implied. Exposure to absolute Input Voltages maximum rating conditions fo
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AD604 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS –DSX1 1 24 VGN1 +DSX1 2 23 VREF PAO1 3 22 OUT1 FBK1 4 21 GND1 PAI1 5 20 VPOS AD604 COM1 6 19 VNEG TOP VIEW COM2 7 18 VNEG (Not to Scale) PAI2 8 17 VPOS FBK2 9 16 GND2 PAO2 10 15 OUT2 +DSX2 11 14 VOCM –DSX2 12 13 VGN2 Figure 2. Pin Configuration Table 3. Pin Function Descriptions Pin No. Mnemonic Description 1 –DSX1 Channel 1 Negative Signal Input to DSX1. 2 +DSX1 Channel 1 Positive Signal Input to DSX1. 3 PAO1 Channel 1 Pr
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AD604 TYPICAL PERFORMANCE CHARACTERISTICS Unless otherwise noted, G (preamplifier) = 14 dB, VREF = 2.5 V (20 dB/V scaling), f = 1 MHz, RL = 500 Ω, CL = 5 pF, TA = 25°C, and VSS = ±5 V. 50 40.0 37.5 40 THEORETICAL 3 CURVES 35.0 –40°C, +25°C, 30 +85°C 32.5 ACTUAL 20 30.0 27.5 10 25.0 0 22.5 –10 20.0 0.1 0.5 0.9 1.3 1.7 2.1 2.5 2.9 1.25 1.50 1.75 2.00 2.25 2.50 VGN (V) VREF (V) Figure 3. Gain vs. VGN for Three Temperatures Figure 6. Gain Scaling vs. VREF 60 2.0 G (PREAMP) = +14d
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AD604 50 2.0 VGN = 2.5V 40 VGN = 2.9V 1.5 30 1.0 VGN = 1.5V 20 20dB/V 0.5 VREF = 2.5V 10 VGN = 0.5V 0 0 VGN = 0.1V –10 –0.5 30dB/V –20 VREF = 1.67V –1.0 –30 VGN = 0V –1.5 –40 –2.0 –50 0.2 0.7 1.2 1.7 2.2 2.7 100k 1M 10M 100M VGN (V) FREQUENCY (Hz) Figure 9. Gain Error vs. VGN for Two Gain Scaling Values Figure 12. AC Response for Various Values of VGN 25 2.55 N= 50 VOCM = 2.5V VGN1 = 1.0V 2.54 VGN2 = 1.0V ΔG(dB) = –40°C 20 G(CH1) – G(CH2) 2.53 2.52 15 2.51 2.50 +25°C 10 2.49 2.48 5 2.47 +85°
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AD604 1000 10 VGN = 2.9V 100 10 1 R ALONE 1 SOURCE 0.1 0.1 0.1 0.5 0.9 1.3 1.7 2.1 2.5 2.9 110 100 1k VGN (V) R ( Ω) SOURCE Figure 15. Input Referred Noise vs. VGN Figure 18. Input Referred Noise vs. RSOURCE 16 900 VGN = 2.9V VGN = 2.9V 15 14 850 13 12 800 11 10 9 750 8 7 700 6 5 4 650 3 2 600 1 –40 –20 0 20 40 60 80 90 1 10 100 1k 10k TEMPERATURE (°C) R ( Ω) SOURCE Figure 16. Input Referred Noise vs. Temperature Figure 19. Noise Figure vs. R SOURCE 40 770 VGN = 2.9V R = 240 Ω S 35 765 30
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AD604 –40 –20 V =1Vp-p V =1V p-p O O VGN = 1V VGN = 1V –30 –45 –40 –50 –50 –60 HD2 –70 –55 –80 HD3 –60 –90 –100 –65 –110 –120 –70 100k 1M 10M 100M 9.96 9.98 10.00 10.02 10.04 FREQUENCY (MHz) FREQUENCY (Hz) Figure 24. Intermodulation Distortion Figure 21. Harmonic Distortion vs. Frequency –30 5 V =1Vp-p O –35 0 –40 INPUT HD2 (10MHz) SIGNAL –5 LIMIT –45 800mV p-p 10MHz –10 –50 –55 –15 HD3 (10MHz) –60 –20 1MHz –65 –25 –70 HD2 (1MHz) –30 HD3 (1MHz) –75 –80 –35 0.5 0.9 1.3 1.7 2.1 2.5 2.9 0.1 0
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AD604 2V V =2Vp-p O VGN = 1.5V 500mV 2.9V 100 90 10 0% 0.1V 500mV 100ns –2V 253ns 100ns/DIV 1.253µs Figure 27. Large Signal Pulse Response Figure 30. Gain Response 200 0 VGN1 = 1V V =200mV p-p O V =1V p-p VGN = 1.5V OUT1 V = GND –10 IN2 –20 VGN2 = 2.9V –30 VGN2 = 2V –40 –50 TRIG'D VGN2 = 1.5V –60 VGN2 = 0.1V –200 –70 100k 1M 10M 100M 253ns 100ns/DIV 1.253µs FREQUENCY (Hz) Figure 28. Small Signal Pulse Response Figure 31. Crosstalk (Channel 1 to Channel 2) vs. Frequency 0 500mV –10 2.9V
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AD604 1M 40 +I (AD604) = +I (PA) + +I (DSX) S S S –I (AD604) = –I (PA) S S 35 100k 30 AD604 (+I ) S 10k 25 DSX (+I ) S 1k 20 15 100 10 PREAMP (±I ) S 10 +I (VGN = 0) S 5 0 1 1k 10k 100k 1M 10M 100M –40 –20 0 20 40 60 80 90 TEMPERATURE (°C) FREQUENCY (Hz) Figure 33. Input Impedance vs. Frequency Figure 35. Supply Current (One Channel) vs. Temperature 27.6 20 27.4 18 27.2 16 27.0 14 26.8 VGN = 0.1V 26.6 12 26.4 10 26.2 VGN = 2.9V 8 26.0 25.8 6 –40 –20 0 2040 608090 100k 1M 10M 100M TEMPERATU
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AD604 THEORY OF OPERATION The AD604 is a dual-channel VGA with an ultralow noise example, if the preamp gain is set to 14 dB and VREF is set to preamplifier. Figure 37 shows the simplified block diagram of 2.50 V (to establish a gain scaling of 20 dB/V), the gain equation one channel. Each identical channel consists of a preamplifier simplifies to with gain setting resistors (R5, R6, and R7) and a single-supply G (dB) = 20 (dB/V) × VGN (V) – 5 dB X-AMP® (hereafter called DSX, differenti
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AD604 preamplifier to be 17.7 dB. The −3 dB small signal bandwidth of PREAMPLIFIER one complete channel of the AD604 (preamplifier and DSX) is The input capability of the following single-supply DSX (2.5 ± 2 V 40 MHz and is independent of gain. for a +5 V supply) limits the maximum input voltage of the preamplifier to ±400 mV for the 14 dB gain configuration or To achieve optimum specifications, power and ground manage- ±200 mV for the 20 dB gain configuration. ment are critical to the
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AD604 The larger portion of the input referred voltage noise comes A unique circuit technique is used to interpolate continuously from the amplifier with 0.63 nV/√Hz. The current noise is among the tap points, thereby providing continuous attenuation independent of gain and depends only on the bias current in from 0 dB to −48.36 dB. The ladder network, together with the the input stage of the preamplifier, which is 3 pA/√Hz. interpolation mechanism, can be considered a voltage-controlled
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AD604 From these equations, it can be seen that all gain curves intercept at AC COUPLING the same −5 dB point; this intercept is +6 dB higher (+1 dB) if The DSX portion of the AD604 is a single-supply circuit and, the preamplifier gain is set to +20 dB or +14 dB lower (−19 dB) therefore, its inputs need to be ac-coupled to accommodate if the preamplifier is not used at all. Outside the central linear ground-based signals. External Capacitors C1 and C2 in Figure 37 range, the gain star
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AD604 The AFA offers the following additional features: Under normal operating conditions, it is best to connect a decoupling capacitor to VOCM, in which case, the common- • The ability to invert the signal by switching the positive mode voltage of the DSX is half the supply voltage, which allows and negative inputs to the ladder network for maximum signal swing. Nevertheless, the common-mode • The possibility of using DSX1 input as a second signal voltage can be shifted up or down by
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AD604 APPLICATIONS INFORMATION The basic circuit in Figure 43 shows the connections for one VREF requires a voltage of 1.25 V to 2.5 V, with between 40 dB/V channel of the AD604. The signal is applied at Pin 5. RGN is and 20 dB/V gain scaling, respectively. Voltage VGN controls normally 0, in which case the preamplifier is set to a gain of 5 the gain; its nominal operating range is from 0.25 V to 2.65 V (14 dB). When FBK1 is left open, the preamplifier is set to a for 20 dB/V gain scaling
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AD604 C1 0.1µF 1 –DSX1 VGN1 24 2 +DSX1 VREF 23 VREF C2 AD604 0.1µF 3 22 PAO1 OUT1 VSET (<0V) 4 FBK1 GND1 21 5 20 VIN PAI1 VPOS +5V R8 C11 R1 (MAX 2k Ω 1µF 49.9 Ω 2 800mV p-p) –(V1) 6 COM1 VNEG 19 –5V R4 1V C8 LOW- 2k Ω 0.33µF PASS 7 COM2 VNEG 18 –5V OFFS FILTER 1 8 NC NULL +5V 8 17 PAI2 VPOS +5V V1 = V ×G 2 +V 7 +5V R7 S IN 87 6 5 1k Ω AD711 9 FBK2 GND2 16 C7 X1 X2 VP W 3 OUT 6 VG 0.33µF C10 10 15 1µF C3 PAO2 OUT2 AD835 OFFS 4 5 –5V –V S 0.1µF C7 NULL R3 0.1µF C6 1k Ω 11 +DSX2 VOCM 14 Y1 Y2 V
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AD604 The 50 Ω termination resistor, in parallel with the 50 Ω source the incoming signal frequency, while passing the low frequency resistance of the signal generator, forms an effective resistance of AM information. The following integrator with a time constant of 25 Ω as seen by the input of the preamplifier, creating 4.07 μV of 2 ms set by R8 and C11 integrates the error signal presented by the low-pass filter and changes VG until the error signal is equal rms noise at a bandwidth of
