Interfacing Multi Channel Sensors over Standard Two Wire Buses
Multi-channel sensor integration on standard two-wire buses demands balancing parasitic bus capacitance, address resolution, layout geometry, and landed unit cost.
PCA9615
Bus Extension
Industrial environments demand reliable methods to extend the reach of standard two-wire communication interfaces beyond their typical circuit board limits. The PCA9615 is a dual-channel multipoint differential bus buffer that converts single-ended clock and data lines into differential signals. This conversion enables I2C communication to operate reliably over long cables up to three meters in length.
By utilizing differential signaling, the device overcomes the distance limitations imposed by standard single-ended bus line capacitance.
Differential Drive
Translating standard open-drain lines to differential pairs requires active driving circuitry. The chip generates complementary outputs for both the clock and the data lines. This differential drive enables the signals to travel over inexpensive twisted-pair cables.
When the signals reach the remote receiver, the chip converts them back to standard single-ended I2C signals. This process occurs transparently to both the master and slave devices on the bus.
Noise Elimination
Electromagnetic noise from electric motors or industrial switchgear presents a constant threat to signal integrity. Because the differential lines run close together in a twisted-pair configuration, any external interference couples equally into both wires of the pair. The differential receiver on the chip detects only the voltage difference, which completely cancels out the common-mode noise.
This ensures high reliability in harsh environments that would otherwise corrupt standard single-ended communication signals.
Transmission Limit
Characterizing the performance of the differential buffer involves measuring the maximum allowable clock frequency against the cable length. This parameter is verified by setting up a test circuit with a thirty-meter Category five cable. Oscilloscope measurements of the differential eye diagram must confirm that the signal transitions occur within the timing requirements of the fast-mode I2C standard.