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We are designing a benchmark circuit for testing diagnosis algorithms. We would like to use the analog switches array AD75019 to build several circuit estructures. But we need to know the maximum current allowed through the switch, before buying the AD75019.
The THD and frequency response specifications on the AD75019 assume a load resistance of 10kohms and a source resistance of 600ohms. Assuming +/-12V supplies, input signal swing of +/-12V and a typical on resistance of 150ohms, the typical input current is 1.12mA. If you reduce the load resistance, then the insertion loss of the switch increases the -3dB bandwidth will reduce, and the THD will degrade. The absolute maximum input current is limited by the power dissipation of the package. The 44pin PLCC package can dissipate 1W absolute Maximum. I would recommend that you operate the device at 75% of it's absolute maximum power rating 750mW. To calculate the maximum current for each switch, we must consider all power dissipated within the AD75019 package. I will assume VDD = +12V, VSS = -12V, VCC = +5V. We must calculate the power dissipated by the analog and digital circuitry, subtract this from the maximum power rating to find how much power can be safely dissipated by all switches. Analog Supplies: +/-12V supplies, with a serial clock of 5MHz, analog supply current is 1mA. Power dissipated by the analog circuitry is then 24V x 1mA = 24mW Digital Supplies: +5V supply, with a serial clock of 5Mhz, digital supply current is 800uA. Power dissipated by digital circuitry is 5V x 800uA = 4mW CMOS switches: The CMOS switches can safely dissipate 750mW - 24mW - 4mW = 722mW Each CMOS switch will dissipate power equal to: P_one_switch = Iswitch x Iswitch x Ron Assuming a maximum of 16 CMOS switches will be on at any time, the power dissipated by the switches is: P_all_switches = 16 x Iswitch x Iswitch x Ron For this calculation we must use the maximum on resistance of 300ohms (assuming +/-12V supplies). P_all_switches = 16 x Iswitch x Iswitch x 300 Re-arranging for Iswitch. Iswitch = sqrt ( 0.722 / [16 x 300] ) Iswitch = 12.3mA The maximum current for each switch should therefore be limited to 12.3mA. From the calculations above, you should be able to calculate the max current for different supplies, and any number of on switches. For example if you could guarantee that only one switch would be on at any given time, a single switch could dissipate 722mW equivalent to 49mA and still be inside the max power rating. Note that the thermal resistance of the 44pin PLCC is approx 40degC/Watt. You should expect the case temperature to rise by 30degC above ambient when dissipating 750mW. One word of warning however. All power dissipation calculations have assumed steady state conditions. Die temperature is a slow moving quantity with time constants of the order of seconds. Do not assume that you can allow a single switch to carry 49mA and then quickly switch to 16 switches carrying 12.3mA. This would likely cause the die to heat up beyond it's max temperature of 115degC. Note on general AD75019 operation: The power up configuration of the AD75019 is indeterminate (not guaranteed). You have to shift configuration data into the part first. Also pin #39 on the AD75019 which is marked as a NC (no connect) on the datasheet should be connected to an analog ground. This pin is actually connected to a shield that should reduce crosstalk.