Q
I'm going to use AD7835 for new design. In our power supply sources, we can
only give out +-12V which is different from the requirement(+-15V 5%) of AD7835
in the datasheet. My question is if I use +-12V and reference 5V to operate
AD7835, Is there any bad influence for this DAC output?
A
The first thing I should say is that the AD7835 is only specified with supplies
of +/-15V, so the operation under any other supplies is not guaranteed or
specified. The following comments are typical of device performance at +/-12V,
not guaranteed, and you should verify these with your own testing.
We know from previous enquiries that the AD7835 will function with supplies of
+/-12V. However, there are a few potential problems at +/-12V which you should
be aware of.
1. POWER ON RESET CIRCUIT
The AD7835 includes a power-on reset circuit which is triggered when the supply
drops to between 10V and 11 V. These figures are typical and aren't tested in
production. I don't know if the POR circuit operates from Vdd or Vss or both so
I'll consider Vdd and Vss to be equal in amplitude and opposite in polarity.
With a +/-15V supply this leaves a large margin for power supply error (4.0V
below nominal voltage)
A +/-12V supply must be more accurate to avoid triggering the reset circuit. If
supply drops below 11V then the DAC reset circuit will be triggered. If the
supply doesn't exceed 11V when powered up then the device may not be properly
reset and DAC registers will remain permanently at zero.
If you test the AD7835 in your system and are satisfied that the power supply
is accurate enough under all operating conditions, then the device will be
functional at +/-12V.
2. OUTPUT STAGE HEADROOM
The AD7835 output stage requires a headroom of 6V from Vdd and Vss. With +/-12V
supplies the output voltage should not exceed +/-6V, to avoid clipping. The
reference input voltage is therefore limited to +/-6V.
(Vref+) - (Vref-) > 5V for specified performance.
If (Vref+) - (Vref-) < 5V, the LSBs will begin to disappear under the noise, so
you no longer have 14-bits noise-free resolution.
As a final comment, the AD7841 octal DAC can be operated at +/-12V, and note 4
on the AD7841 datasheet explains the restrictions when operating the AD7841
with +/-12V supplies.