4 data bits can attenuate 0~15DB programmable attenuator

4 data bits can attenuate 0~15DB programmable attenuator

Function of the circuit

Most electronic devices usually require variable gain amplifiers. At low frequencies, a variable gain amplifier with arbitrary gain can be constructed by changing the feedback degree of the OP amplifier. However, with this method, since the open-loop frequency characteristics remain unchanged, once the gain is increased, the high-frequency characteristics will change. If the phase characteristics are considered, the frequency range of use will be limited.

This circuit uses a fixed gain amplifier, and only adds a resistor attenuator at the input end, which limits the attenuator to change significantly, and the frequency characteristics remain basically unchanged. This circuit can use TTL level, and after weighting 1, 2, 4, 8, it can form an attenuator that can directly read data. When the selected data is 0~F, the attenuation is 0.~15DB.

How the Circuit Works

This circuit uses a relay with contacts to switch fixed attenuators in sequence to obtain stable attenuation. Usually, π-type and T-type attenuators are more commonly used. This circuit uses a π-type circuit with symmetrical input and output. Attenuators can be divided into balanced and unbalanced. When connected to electronic circuits, unbalanced attenuators can be used.

This circuit is designed for a terminal impedance of 600 ohms. If it is not 600 ohms, the correct attenuator cannot be obtained.

The relay uses the double-contact G5A237P for small signal springs, and is switched on and off by a Darlington driver. The coil of each relay is connected in parallel with a diode that absorbs reverse voltage. If the diode is not grounded, a spike voltage will be generated at both ends of the coil and noise will be mixed into the signal.

The resistance value that determines the attenuation of the π-type attenuator can be calculated using the following formula; let the terminal resistance be R. ,

The X in the formula represents the attenuation, the unit is DB.

Table A lists the calculated values ​​of R1 and R2 when the attenuation is 0.1DB~80DB (left column). Unless customized, it is difficult to find all these resistance values ​​in the series resistors. Therefore, we selected the resistance values ​​close to them from the E96 series products and listed them in the table (right column). The biggest difference is 20DB 2.97K. In this case, it is better to use a 3K and a 300K resistor in parallel instead of 2.94K.

If a first-stage attenuator is used to attenuate 80DB, the parasitic capacitance of the relay will deteriorate the high-frequency characteristics. Two groups should be connected in series, with each group attenuating 40DB.

Installation Notes

For attenuators with large attenuation, in order to reduce the coupling between the output and input, isolation should be added between the output and input, and good grounding should be provided.

Because the signal level will be reduced after the attenuator, in order to avoid interference, it is best to install the entire circuit in a shielding box.