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Oct 15 2020

AD603 Variable Gain Amplifier: Pinout, Datasheet, Circuit

I.Introduction

The AD603 is a low noise, voltage-controlled amplifier for use in RF and IF AGC systems. It provides accurate, pin-selectable gains of −11 dB to +31 dB with a bandwidth of 90 MHz or +9 dB to 51+ dB with a bandwidth of 9 MHz. Any intermediate gain range may be arranged using one external resistor. The input referred noise spectral density is only 1.3 nV/√Hz, and power consumption is 125 mW at the recommended ±5 V supplies.

AD603

Catalog

I.Introduction

II.Features

III. Documents and Media

IV. Pinout Configuration

V. Functional Block Diagram

VI. AD603 Working Modes

VII. Typical Application

VIII. Applications

IX. Application Note

FAQ


II.Features

  • Linear-in-dB gain control
  • Pin-programmable gain ranges:  −11 dB to +31 dB with 90 MHz bandwidth

                                                              9 dB to 51 dB with 9 MHz bandwidth

  • Any intermediate range, for example −1 dB to +41 dB with 30 MHz bandwidth
  • Bandwidth independent of variable gain
  • 1.3 nV/√Hz input noise spectral density
  • 6 ±0.5 dB typical gain accuracy

III. Documents and Media

Datasheet

AD603 Amplifier Datasheet


IV.Pinout Configuration

ad603 pinout

ad603 pin function


V. Functional Block Diagram

ad603 functional block diagram

Figure 1 AD603 functional block diagram

It is not difficult to find that it is different from AD600 in that: the fixed gain amplifier it uses can change the gain value. The gain GF is determined by the connection form of VOUT and FDBK. When VOUT and FDBK are short-circuited, GF=31.07dB; when it is open, GF=51.07dB; connect resistor REXT between VOUT and FDBK to set GF Any value between 31.07dB~51.07dB. However, the gain accuracy in this mode is reduced. When the external resistance is about 2K, the error is the largest. If an appropriate resistor is connected between VOUT and COMM, the gain can be increased, up to 60dB.


VI. AD603 Working Modes

AD603 has three working modes:

Mode 1: Short-circuit VOUT and FDBK, this connection can obtain the maximum bandwidth-90 MHz, and the gain range is -11.07dB~+31.07dB. As shown in Figure 2.

Figure 2 Short connection between VOUT and FDBK

Figure 2 Short connection between VOUT and FDBK

Mode 2: Connect a resistor REXT between VOUT and FDBK, and a 5.6pF capacitor between FDBK and COMM as frequency compensation. According to the relational expression of the amplifier, selecting the appropriate REXT value can obtain different gain range values. When REXT=2.15K ohms, the gain range is: -1dB~+41dB. As shown in Figure 3.

Figure 3 VOUT and FDBK access resistance REXT

Figure 3 VOUT and FDBK access resistance REXT

Mode 3: Open a circuit between VOUT and FDBK, and connect an 18pF capacitor between VOUT and COMM to extend the frequency response range. This mode is a high gain mode with a gain range of 8.93dB~51.07dB and a bandwidth of 9MHz. As shown in Figure 4.

Figure 4 High gain mode

Figure 4 High gain mode

In the above three modes, the relationship between gain GF and control voltage VG is shown in Figure 5.

Figure 5 The relationship between gain GF and control voltage VG

Figure 5 The relationship between gain GF and control voltage VG

When VG is in the range of -500mV~+500mV at 40dB/V (that is 25mV/dB, which is different from AD600's 32mV/dB) for linear gain control, the relationship between gain G (dB) and VG (V) is: G =40VG+Goi(I=1, 2, 3), where VG=VPOS-VNEG. G0i is the different gain constants in three modes. Mode 1: GOi=10dB; Mode 2: GOi=10dB~30dB (determined by the external resistor REXT); Mode 3: GOi=30dB.

When the control voltage VG is outside -500mV~+500mV, the gain G and VG no longer satisfy the linear relationship. When VG=-526mV, the gain is G=GF-42.14, when VG=+526, the gain is G= GF.


VII. Typical Application

Figure 6 AD603 typical application circuit

Figure 6 AD603 typical application circuit

Figure 6 is a two-stage AD603 amplifier circuit with automatic gain control. In the figure, Q1 and R8 form a detector to detect changes in the amplitude of the output signal. The automatic gain control voltage VAGC is formed by CAV, the difference between the current Q2 and the collector current of Q1 flowing into the capacitor CAV, and its magnitude changes with the amplitude of the output signal of A2, which makes it added to A1 and A2 amplifier 1. The automatic gain control voltage VAGC of the pin changes with the output signal amplitude change, so as to achieve the purpose of automatically adjusting the amplifier gain.


VIII. Applications

  • RF/IF AGC amplifiers
  • Video gain controls
  • A/D range extensions
  • Signal measurements

IX. Application Note

(1) The power supply voltage should generally be selected as ±5V, and the maximum should not exceed ±7.5V.

 

(2) In the case of ±5V power supply, the effective value of the rated voltage applied to the input terminal VINP should be 1V, the peak value is ±1.4V, and the maximum should not exceed ±2V. If you want to expand the measurement range, you should add a level of attenuation in front of AD603. In this way, the typical value of the peak output voltage can reach ±3.0V. Therefore, it is usually necessary to add a first level of amplification after AD603 to connect to the A/D converter.

 

(3) The voltage applied to the voltage control terminal must be very stable, otherwise the gain will be unstable, which will increase the noise of the amplified signal.

 

(4) The signal must be directly connected to pin 4 of the amplifier, otherwise the accuracy of the amplifier will be reduced due to the large impedance.


FAQ

  • What is AD603?

AD603 is a low-noise, voltage-controlled amplifier for radio frequency (RF) and intermediate frequency (IF) automatic gain control (AGC) systems. It provides precise pin-selectable gain, with a gain range of -11 dB to +31 dB at 90 MHz bandwidth, and a gain range of +9 dB to +51 dB at 9 MHz bandwidth. Any intermediate gain range can be obtained with an external resistor. The noise spectral density referred to the input is only 1.3 nV/√Hz, and the power consumption is 125mW when using the recommended ±5 V power supply.

  • What are the problems that need to be paid attention to when using AD603?

The voltage cannot be too high. Generally, the voltage is plus or minus 5V, and the maximum voltage cannot exceed plus or minus 7.5V. The output voltage cannot exceed 2V.

  • How to solve the self-oscillation problem of AD603?

For high-frequency operational amplifiers, the following points are the basic ways to solve self-excitation.

  1. The power supply is stable and no ripple.
  2. The electrical connection wires are as short as possible.
  3. The ad603 circuit should be far away from the power circuit, especially away from the transformer.
  4. The power transformer and the circuit board of ad603 should be shielded with a metal box and grounded if possible.
  5. One point is very important. For op amps, too large magnification can easily cause self-excitation, so reduce the magnification as much as possible and minimize the number of magnification levels (generally not greater than 4).
  6. Reverse amplification can suppress self-excitation in multi-stage amplification.
  7. If you want to connect to the power amplifier and then amplify, it is best to use two power supplies, and the circuit should be connected to the same ground.
  • What is the difference between AD603AQ and AD603AR?

Their differences are in model, Temperature, Package.

AD603AQ -40°C to +85°C 8-Lead CERDIP

AD603AR -40°C to +85°C 8-Lead SOIC_N

  • After inputting an AC signal and being amplified by AD603, why does the output contain a DC signal? How to eliminate the DC signal?

When the DC blocking capacitor is not used, the bias voltage of the input circuit needs to be adjusted for compensation.

If the DC voltage of the AC signal is not fixed, only a DC blocking capacitor can be used, or the average value can be used to eliminate it after sampling the number.

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