Part Number: DAC7678
I am using eight DAC7678 in a system. The addresses are set for each one by jumpers on the card. All addresses for which the address pins are pulled high or low work properly. All addresses for which one pin is left open do not. They behave as if the pin is pulled high. What is the expected input voltage on a pin which is left floating? How can I fix this issue?
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In reply to Uttam Sahu:
They all measure 5V when open.
In reply to David Siddons:
On further investigation I discovered that the address pins had been installed with pull-ups to 5V, which explains that. Sorry for that misinformation. However, when I removed the pull-ups, the "floating" pins now measure 0V, and the addresses follow that.
I also tried adding a 10k resistor divider to the address pins, and verified that when left open they sit at 2.5V. Under these conditions, the addresses look like zeros when left floating at 2.5V.
Has anyone else tried to use these "floating" addresses for this device?
I see two values, depending on the polarity of the multimeter. WIth the multimeter red probe on the pin, I see 560kohm to both GND and VDD. With reversed probes I see 270kohm to both GND and VDD.
That is quite a lot of stuff ! It will be a while before I can get to it. The project consists of eight daughter cards supervised by a ZynQ-based controller. I have attached a picture of the daughter board. Conn6 and Conn7 are the DAC address jumpers, and the chip is just to the right of them. R28 & R29 were the pull-ups which I have removed. Luckily for me, I had provided for standalone operation of the board by providing trimmers for each of the values to be controlled by the DAC, so I can work with them, however painfully!
I have no control over the power supply turn-on rate. It is a small 5V switcher module. I can easily imagine it takes longer than 1ms for it to come up, so to make the test you propose will be a project for me. I will ask around to see if something is available which could test the hypothesis.
OK, I removed the chip. Both inputs now measure open circuit.
Before I removed it, I re-checked the voltage using the 'scope probe rather than the multimeter. Still shows 0V.
The power-on takes about 4ms.
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