ADS8509: When the power is turned on, the RC pin voltage rises slowly, so it enters a mode in which BUSY continues to be output.

Hello,

The R/C pin on the ADS8509 is a digital input pin, which means the host device is controlling it. This is not an output from the device, thus if the RC signal is not satisfactory, the host device (microcontroller or similar) is the source of the issue, this is where to look. 

As the pin description states below, the state of the pin can initiate a conversion, resulting in BUSY being active, the device seems to be responding as expected. 

I would suggest that during start up, the host device keeps the RC pin high until the system has fully powered on and settled. From there, the host can begin to communicate with the device and begin conversions. 

If you would like to share a schematic, we can review it. 

Regards

Cynthia

Hi Cynthia

A schematic and timing diagram are shown below.

The voltage at VDD+3.3V is stable.
Then, when the VDD+5V power supply is turned on, the VDD+5V supplied to the ADS8509 gradually increases, but the R/C signal rises to VDD+5V when power supply becomes 2.5V because of  ISO7240M.

This causes the ADS8509 to output BUSY signal.
The issue is that this BUSY signal stays low and never returns to high.
In the datasheet, tw2 is max 2.2us, but BUSY signal never returns to High.
How would I reset the ADS8509 externally?

Best regards
k.nishizawa

Can you clarify, is RC pin up to 3.3V before the device is powered up? If so, this is in violation of the abs max, no digital pin should be greater than Vdig of the device. Though this is likely not causing damage to the device due to the short amount of time it is in this state, I would suggest addressing this.

This device does not have a demanding power up sequence, but I suspect there is something unexpected behavior in the ramp up. Can you watch the other pins on the device as well during this time, in particular, CS.

Have you tried the PWRD pin, this will power down the analog circuitry. 

Regards

Cynthia

Hi Cynthia

RC pin is below 3.3V before the device is powered up, because VCCO of ISO7240M is connected to power source of ADS8509.

CS pin and PWRD pin are fixed to GND.

As shown in the figure, when the RC signal goes low, the BUSY signal goes high.

Since the BUSY signal has returned to high, is it okay to continue the normal sequence after this?

Regards

K.Nishizawa

Hello,

Have you had an opportunity to move forward with this?

I would suggest starting the device with RC in a high state, and not in a low state as you are now. This can be done with a pull up resistor, or change the state within the host device. The device currently is starting with both CS and RC at low states, this is starting a conversion, even before the ADC is fully powered on and settled. 

With CS and PWRD both hard wired to ground, the only method to communicate with the device to begin conversions is through RC

Regards

Cynthia 

Hello Cynthia

We considered a pull-up resistor to the RC pin before,
in that case, the RC pulse width of ADC sequence also need to be ajusted.

Our plan is following:
Since setting RC to LOW once causes the BUSY pin to go to HIGH, we are changing the FPGA program so that the FPGA executes a sequence that sets RC to LOW once after startup.

Regards
Nishizawa