Question
I have an AD7730BN part connected to a PC printer port (for develop only)
trough a 1K series resistor in SCLK,DIN,DOUT,CS lines. I saw that if I
switch-off the AD7730BN power supply I can't start conversion commands but I
can read/write to the registers, it seams that the digital section of the part
is powered from the data lines. Can be this true?. Some times I found a
residual voltage (some mV.) in the analog input pins Can be this due to this
phantom power supply?
Answer
The AD7730 has ESD protection diodes between the data lines and the DVDD and
GND rails on the chip. This means that in certain cases the digital section of
the device is powered via one of the digital inputs and via the ESD protection
diode.
You should be aware that this may damage the device. The absolute maximum
ratings state that
"Digital Input Voltage to DGND . . . . -0.3 V to DVDD + 0.3 V"
If DVDD is 0V then there shouldn't be any voltage on the digital inputs greater
than -0.3V to +0.3V. The 1k resistors that you have placed in series will
probably prevent any damage occuring to the IC, but it is undesirable to have
the digital section of the IC powered in this unreliable way (power supply is
high impedance and not always present). The AD7730 should therfore be powered
before being plugged into the printer port of the PC.