Galvanic isolation of the video signal is indispensable when creating a video surveillance system, if interference suppression is necessary.
The device allows you to amplify the video signal by smoothly adjusting the gain level with a trimmer resistor and compensating for the loss of image clarity that occurs on long cables.
Galvanic isolation — diagram
The basis of the isolation device for video surveillance systems is a galvanic isolation transformer. In most transformer circuits, the magnetic circuit of the transformer is used to electrically isolate the secondary circuit from the primary circuit.
In the case where the control chip is located on the side of the primary power source, it is required that the output voltage feedback circuit crosses the isolation barrier.
Galvanic isolation — a circuit for an analog signal: the transformer for power transmission is usually excited by rectangular pulses, which are generated by a simple generator.
You can use the ADC clock pulses for this.
This obvious trick allows to get rid of one optocoupler.
The supply voltage is rectified by a diode and smoothed by a capacitor.
The clock pulses are filtered from unwanted surges by the circuit.
Galvanic isolation — the circuit is simplified if a voltage-to-frequency converter is used instead of an ADC. The VFC operates continuously and does not require control.
However, such a solution requires additional data processing (frequency measurement), and this either loads the microcontroller or requires the use of additional specialized microcircuits. This is especially undesirable in multichannel systems.
Using an ADC provides the microcontroller with a digital value without additional processing, but requires, at a minimum, the «Start conversion» command and/or synchronization of the serial interface.
To start the converter, you can use the power monitor.
After applying the supply voltage, the monitor will give the command «Start conversion».
By turning the power on/off for a sufficient time, you can ensure control, but such a scheme worsens such an important system parameter as the number of conversions per unit of time.
The total current consumption of the circuit on the ADC side must be low enough, since during the «pauses» of the clock frequency the entire circuit is powered by the energy stored in the capacitor C2.
To obtain data, it is necessary to find a value, by changing which it is possible to transfer information from the ADC side to the microcontroller.
The 4-20 mA current loop is a good example of data transfer by changing the consumed current.
In our case, this is the only way, with the only difference that the instantaneous consumption of the circuit will change.
If the ADC transmits a logical zero, the transistor is closed, and the circuit's current consumption is equal to the total consumption of all electronic components on the ADC side.
When transmitting a logical one, the transistor opens and adds additional load.
The current consumption sensor and comparator generate serial data for loading into the galvanic isolation unit via the microcontroller.
For normal operation of the circuit, the difference in current consumption without (and with) a resistor must be significant.
The converter has a serial interface, extremely low power consumption and a unique control pin.
The microcircuit is used to generate the ADC reference voltage, as well as to power the entire circuit. The comparator generates clock pulses.
The resistor is used to limit the current through the input protective diodes, including during the initial power supply.
The galvanic isolation transformer must be selected taking into account the need to ensure the absence of saturation when transmitting logical units, as well as a drop in current to zero when the transistor is closed.
The magnetic circuit must not have gaps.
The galvanic isolation unit can accommodate passive input protection, a low-pass filter, and an input divider.
To start the conversion, the microcontroller must skip 7 periods of the clock frequency.
After that, transmitting 12 clocks, it must load the data from the ADC, latching them on the edge of the clock pulses.
After the 12th pulse and until the next conversion, the microcontroller will receive a sequence of logical zeros.
By complicating the control algorithm, it is possible to ensure switching of the inputs, thereby implementing a two-channel isolated measurement circuit.
As a result, it is possible to use only one galvanic isolation transformer for the isolation barrier.
The isolation voltage depends only on the transformer design, it can exceed the most common values of 1.5 kV or 2.5 kV.
The resulting circuit is simple, inexpensive and requires a minimum of space on the printed circuit board. board.
This allows it to be used to implement multi-channel measurement circuits with individual isolation and simultaneous measurement.