The ADAS1000 oscillator circuit requires total capacitance seen at the XTAL pins <20pF (ideally <15pF) to ensure reliable start up across operating conditions.
For some crystal oscillators, the pad footprint can be quite large and if the crystal is placed at a distance from the ADAS1000 device, then there can extra capacitance present at the XTAL pin that could cause issues with startup.
On the ADI evaluation board, the crystal used is a 8.192MHz crystal ECS-81.92-18-7SX-TR
- Rm (ESR) = 100ohms max
- C0 = 7pF max
- Rm range 40 to 70ohms typical
- Cparasitic = 4pF (board/pad/trace capacitance)
- Variables in simulation = Temperature, C1 (crystal load), Co and ESR of crystal
The typical simulation plot below shows the crystal start up time over temperature for Co = 3pF and ESR = 40ohms.
C1 = 6pF (green) and 10pF (yellow).
The startup time increases with increasing temperature and with larger C1 as expected, but always starts up.
The following simulation plots Co = 5pF, ESR = 70ohms, C1 = 6pF (green) and 10pF(yellow)
It can be seen that the oscillator circuit takes longer to start up at higher temperature, but does always start up.
In your application, it is recommended to review the following:
- Assess the crystal parameters, ESR and Co, choose a crystal with low ESR. Many manufacturers will provide test data showing typical crystal performance parameters.
- Review your layout to assess the crystal location and footprint ensuring there is no addition of extra capacitance (total capacitance at the XTAL pin should be <20pF), measure the pin capacitance at the pad if necessary using a cap meter.
- Use crystal load capacitor value from 5.6pF to 10pF
- Using a smaller crystal capacitor value will have a pulling effect on the frequency. The frequency error should be <500ppm.
If you suspect a crystal start up issue in your application (conversions not happening), then it is possible to try measure if the oscillator circuit is powering up.
Note: even if the crystal is not coming up, it is still possible to read/write the registers - so you should be able to confirm the power on default register settings of all registers.
Firstly you need to power the device up and enable it into operating mode (suggest writing 0xF804AE to the ECGCTL register (takes the device out of low power mode, enables the voltage reference). Take care not to connect a standard probe to the XTAL pins as this will load the circuit even more - use active probes.
If an active probe is not available, then write 0xF804BE to the ECGCTL register (takes the device out of low power mode, enables the voltage reference, enables into gang mode as a master which turns on the CLK_IO pin to bring the clock out onto the CLK_IO hardware line), now use a scope probe on the CLK_IO line to see if there is a 8.192MHz signal there.
Example of measurement on ADI evaluation board with C1=C2 =10pF