PCM1804 acting up

[MC]

Heya,

I've got a Texas Instruments PCM1804 analog to digital converter hooked up like so:
http://www.pixelwrench.com/pcm1804.jpg

However, It's in it's "bare minimum" configuration.. I've hooked up the analog and digital voltages, their grounds, set the clock configuration pins, and attached the crystal. When I put my volt meter between the overflow pins (OVFR and OVFL) and ground, I measure about 3 volts. If I place a small LED in place of that, it seems to randomly flicker and dim. I've done this with several PCM1804 chips and I seem to get the same result.

I've tried grounding and/or bringing the analog input pins and this still happens. Is this supposed to happen or am I doing something obvioulsly wrong.

Thank you!

Best Regards,
Matt Carpenter

[rfbrw]

Is the device configured as a master or a slave?

[MC]

Ive got nothing connected to the M/S pin so I'd assume its configured as master.

Ive been informed that the problem could possibly be because I'm not providing a proper clock generator circuit. I've always thought I could hook a crystal up as shown in my schematic and it'd work fine -- infact it does so with my PIC chips, I've never needed any extra circuitry along with my crystal other than a couple caps and a resistor. Could this be the problem? I don't have a scope so I can't really check to see if it's oscillating or not.

[rfbrw]

If the inputs are grounded, Vcom is at circa 1/2 V+ and Vref is decoupled then the input is at max neg and that may well trigger an OVFL/R condition. If OTOH, Vcom and Vref are grounded, all bets are off.

[apollyon25]

There is a fundamental mistake in your circuit.

The clock input is an input, it is typically hiZ (or thereabouts) and is required to be driven. The crystal itself is a PASSIVE device.
You cannot just stick a crystal to a device and expect it to work.

A bit of oscillator and crystal theory:

Oscillators work on the principle of feedback. If you can recall speaker/microphone feedback squeal at live venues... it is due to the signal being picked up by the microphone, amplified by the mixing desk/amplifiers and this amplified signal being fed into the microphone with the same phase as the original signal. These two signals add constructively, resulting in more amplified signal, more signal into the mic, etc, until it begins to squeal.

Crystals use the same principles... however initially the noise generated by electron movement (called shot, johnson or thermal noise depending on device...) is amplified (by crystal Q factor usually)and then fed into the crystal. The crystal resonant frequency is also its oscillation frequency (although they actually have a couple of modes - fundamental and 3rd overtone the most common) and this resonance filters the amplified noise and feeds it across the crystal resulting in a signal of the resonant freq of the crystal... in phase with the original... which get in, amplified, fed back, in, amplified, fed back etc...

You are missing the amplifier circuit to start the crystal!

You will need to add a circuit to drive the SCK pin of the PCM1804. TI make a clock generator specifically for audio circuits eg. PLL1707 (refer fig 8 in the PLL1707/PLL1708 datasheet). This device has your more usual crystal input circuit.

There are loads of circuit topologies for making oscillators (Colpitts, Pierce/Pearce, Hartley, etc) but most use a chip like above... or some form of microcontroller-input-type.

Next time when you come across a problem like this - read the datasheets! Infact read the datasheets BEFORE you start!

Enjoy!

apollyon25

[MC]

Well, I think that certainly describes why it hasn't been working. Thank you for pointing that out

I had read the datasheet beforehand -- however, I wasn't aware of the fact that I need a seperate clock generator circuit. I'm more of a driver programmer and live audio reinforcement technician than an electronics engineer. Thanks for bearing with me

[rfbrw]

If your design has the usual xtal, 2 caps, resistor and an inverter it should be enough.