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I need to run three AD7779 ADCs using the same clock signal for synchronization. I don't have access to an oscillator which I can use as a master clock, so I was wondering how I could use a single passive crystal to run all three ICs.

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  • \$\begingroup\$ I don't think you can. The ADCs can't all be trying to drive the same crystal simultaneously. You need to produce a clock signal somehow first and at a quick glance it does not look like these ADCs have a clock output. If they did it would mean that one ADC could drive the crystal and output the clock which could then be fed to other ADCs. \$\endgroup\$
    – DKNguyen
    Commented May 6, 2023 at 19:43
  • \$\begingroup\$ Tombeki, Isn't there a section in the datasheet about synchronization? What about that doesn't work for you? \$\endgroup\$ Commented May 6, 2023 at 20:23
  • \$\begingroup\$ Why not use Sync_IN and Sync_OUT provided mechanism ? \$\endgroup\$
    – Antonio51
    Commented May 7, 2023 at 6:59

4 Answers 4

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The ADC you describe supports the use of a CMOS clock signal, as well as the use of a passive crystal. While you state that your selection of available components is limited, basic logic gates may be easier to source.

If you use a crystal alongside two inverter logic gates (one unbuffered, one Schmitt-triggered in the example below), you can then drive multiple chips from the output of the second logic gate.

schematic

simulate this circuit – Schematic created using CircuitLab

This diagram is based on TI Application Note SCEA099 (https://www.ti.com/lit/an/scea099/scea099.pdf)

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    \$\begingroup\$ There are two resistors in the schematic of the application note. They might be necessary. \$\endgroup\$
    – Uwe
    Commented May 7, 2023 at 11:46
  • \$\begingroup\$ @Uwe an accidental omission, now rectified, thanks. \$\endgroup\$
    – Kaz
    Commented May 7, 2023 at 14:27
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    \$\begingroup\$ The (SN)74LVC1GX04 combines these two inverters. \$\endgroup\$
    – CL.
    Commented May 7, 2023 at 14:30
  • \$\begingroup\$ @CL. Strange that that's a 1G part and not a 2G one. \$\endgroup\$
    – Hearth
    Commented May 7, 2023 at 14:45
  • \$\begingroup\$ @Hearth Presumably because it doesn't allow you to use them independently: it only has one input pin. \$\endgroup\$
    – Kaz
    Commented May 7, 2023 at 16:52
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I don't have access to an oscillator which I can use as a master clock

I don't quite understand what that means: you have to put an external oscillator in there - how would you not have "access" to it? You'd also need a clock distribution chip. The two can be all on one chip, see eg. the multitude of chips from Skyworx Solutions with several clock outputs.

how can I use a single passive crystal to run all three IC's.

By using a crystal with an oscillator/driver chip that has at least 3 clock outputs. Each output goes to one of the ADCs. Done.

There's a ton of those chips on DigiKey.

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  • \$\begingroup\$ I dont have access to DigiKey due to several reasons. I am limited by the domestic suppliers who dont have oscillators. I can only acquire a passive crystal. I was planning on creating a clock signal with the crystal using extra circuitry, then buffering it using SN74HC125 \$\endgroup\$
    – Tombeki
    Commented May 6, 2023 at 19:53
  • \$\begingroup\$ That could work (a roll your own clock distribution scheme). How well do the clocks into each ADC have to be matched? Is there a phase matching requirement? \$\endgroup\$
    – SteveSh
    Commented May 6, 2023 at 20:17
  • \$\begingroup\$ @Tombeki You can buy an AD7779 and everything else it'll need to be useful but can't buy an oscillator chip? How is that possible? \$\endgroup\$ Commented May 6, 2023 at 20:57
  • \$\begingroup\$ With a 8.192 MHz crystal oscillator you don't need at least 3 clock outputs. A single output will do, routed to the three AD7779 ADCs. The total length of the clock line should be less than 100 mm. Only to achieve a very small phase difference between the ADCs the three clock outputs may be needed. \$\endgroup\$
    – Uwe
    Commented May 7, 2023 at 8:53
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You may be able to tie the crystal output pin XTAL1 through a buffer chip to the MCLK/XTAL2 input on two of the ADCs but I think it might be iffy. There's no guarantee the voltage swing is adequate.

I'd just add the oscillator, personally. It's tiny, cheap (eg. one similar to the 8.192MHz one used in the eval board) and will be more reliable than the crystal. Those are not cheap ADCs, it would be good to get the most out of them.

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One option is to have a basic crystal oscillator circuit similar to what is inside the chip.

Make a Pierce oscillator with simple unbuffered inverter like 74HCU04 or 4069UB. Then you have a square wave out and can run it to two chips.

It might also be possible to use one chip as the oscillator, and then use the oscillator output on one chip to drive the oscillator input on the other chip.

Another option, if you have a MCU in the circuit, is to use a crystal on the MCU and in some way make it output a clock. Some MCUs have specific clock output pins. Some MCUs don't, so you could use hardware PWM output.

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  • \$\begingroup\$ I was just looking at these when your answer appeared. At 3.3V here speed capabilities drop. 8192kHz may be marginal. At lower frequencies or highevoltages they should be fine. \$\endgroup\$
    – RussellH
    Commented May 6, 2023 at 21:00

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