Huge consumption of LT1679

Can you post a picture of your board.

Have you simulated the design in LTSpice ?

Where did you get the phase splitter ADC driver circuit from ?

MK

Can you post a picture of your board.

Here it is.

In the actual prototype I added a 1 uF cap placed very close to the ADM660 power supply pin.

Have you simulated the design in LTSpice ?

Yes, I attach the file.

I've just run an AC simulation: I got a peak in the phase at 800 kHz (which is exactly the frequency of the oscillation I noticed before). With a phase of 6° and a gain of 3.9 dB. Is it enough to sustain such an oscillation?

Where did you get the phase splitter ADC driver circuit from ?

From CN0048 of Analog Device:

http://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0048.pdf

Attachments:

front-end.asc.zip

I don't much like the ground fill around the amplifier chip because it adds distributed stray capacitance to slightly imperfect ground which is hard to simulate. However sticking small caps to ground in the simulation has little effect so there is no evidence that the ground fill is the problem. I prefer using four layers which doesn't reduce the stray capacity to ground but gives you a much better reference to decouple to.

Since you have aboard you can try poking little caps across various places until it works, or try a different op amp. The very low resistor values will not help in terms of power consumption  - it might be worth experimenting with increasing them.

MK

I wasn't able to stop the oscillation placing caps around.

Changing the op-amp with an MC33174 leads to no more oscillation and the current consumption is about 30 mA (now I agree the values of resistors won't help...)

The downside is that with MC33174 the circuit doesn't work properly - I mean the output is not correct. One side goes negative and the other change just a little bit around 1 V. I increased the supply voltage to ±6.5 V but changes nothing.

What Vcc and Vee are you using  - the MC33174 is nothing like rail to rail operating on input or output.

MK

What Vcc and Vee are you using  - the MC33174 is nothing like rail to rail operating on input or output.

MK

I wrote it in the previous post: ±6.5 V. The input is clamped at ±4.7 V. The outputs are expected to be in between 0.6 V and 3.1 V

So you did !

(I was looking at the circuit.)

The MC33174 should sort of work (but not very good drift and offset specs.) I'm wondering if there is something wrong with the board - short or missing connection.

It's v tedious to do but I suggest that you set up LTSpice for DC simulation with a 1V DC input and compare voltages on your actual circuit (with 33174s) with the sim.

Perhaps you'll spot something there.

MK

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