Q
Our application will be using Vdd= 5V for the AD7450. According to specs:
"logic-input-voltage-high Vinh=2.4 Vmin" Is this value valid for the entire
Vdd range from 2.7 to 5.25V or just for the lowest voltage of Vdd=2.7V? What is
max logic-output-high-voltage, given Vdd=5V? Can this voltage exceed 3.6V?
(In other words: can the ADC be interfaced directly to 3.3V DSP ports without
any buffers?)
A
You are correct that for logic inputs, the spec is over the entire supply range.
For the logic outputs, when operating the AD7450 with a 5V supply, the maximum
digital output high voltage is actually VDD. So, unfortunately, it would be be
not be possible to directly interface the logic outputs of the AD7450 to a 3.3
V processor. Some extra circuitry will be required; series resistor plus
Schottky diode to the 3.3V supply, or level translator chip.
The way we solve this problem on our ADCs is by using a separate "Vdrive" pin
to power the digital interface. You can see this feature on the AD7490 for
example. On the AD7450 we went for small package size and this just wasn't a
spare pin for the Vdrive function.
The alternative would be to change the AD7450 supply to 3.3V but I am not sure
if this is an option.
