Internal structure, principle and precautions of AD603

Internal structure and principle of AD603

The simplified principle block diagram of AD603 is shown in Figure 2. It consists of three parts: passive input attenuator, gain control interface and fixed gain amplifier. In the figure, the signal added to the input end (V INP) of the ladder network is attenuated and output by the fixed gain amplifier. The attenuation is determined by the voltage added to the gain control interface. The gain adjustment has nothing to do with its own voltage value, but only with its difference VG. Since the input resistance of the control voltage GPOS/GNEG end is as high as 50MΩ, the input current is very small, which reduces the influence of the on-chip control circuit on the external circuit that provides the gain control voltage. The above characteristics are very suitable for forming a programmable gain amplifier. The "slider" in Figure 2 can be moved continuously from left to right. When the connection of the two pins VOU T and FDB K is different, the gain range of the amplifier is also different.
When pins 5 and 7 are short-circuited, the gain of AD603 is 40Vg + 10, and the gain range at this time is -10 to +30dB. When pin 5 and pin 7 are disconnected, the gain is 40Vg + 30, and the gain range is 10-50dB. If resistors are connected to pins 5 and 7, the gain range will be between the above two.
The input impedance of the gain control interface of AD603 is very high. In multi-channel or cascade applications, one control voltage can drive multiple op amps; at the same time, its gain control interface also has differential input capability, and the appropriate control scheme can be selected according to the signal level and polarity during design.

Use of AD603 Precautions
In the application of AD603, the following points should be noted:
(1) The power supply voltage should generally be selected as ±5V, and the maximum should not exceed ±7.5V.
(2) Under the condition of ±5V power supply, the rated voltage applied to the input terminal V INP should be 1V with a peak value of ±1.4V and a maximum of ±2V. If the measurement range is to be expanded, a first attenuation should be added in front of AD603. This can make the typical value of the output voltage peak reach ±3.0V. Therefore, a first amplification stage is usually added after AD603 before it can be connected to the A/D converter.
(3) The voltage applied to the voltage control terminal must be very stable, otherwise it will cause gain instability, thereby increasing the noise of the amplified signal.
(4) The signal ground must be directly connected to pin 4 of the amplifier, otherwise the amplifier accuracy will be reduced due to the large impedance.