Driving a coil

[Mizha Mekka]

Hi everyone!

My coil has a resistance of 24 Ohm and a inductance of 13mH, and I'm using DAC8581 from Texas Instrument to drive the coil, the datasheet suggests:

The DAC8581 uses a high-performance, rail-to-rail output buffer capable of driving a 600-Ω, 200-pF load with fast 0.65-μs settling. The buffer has exceptional noise performance (20 nV/√Hz) and fast slew rate (35 V/μs). The small-signal settling time is under 300 ns, allowing update rates up to 3 MSPS. Loads of 50 Ω or 75 Ω could be driven as long as output
current does not exceed ±25 mA continuously. Long cables, up to 1 nF in capacitance, can be driven without the use of external buffers. To aid stability under large capacitive loads (>1 nF), a small series resistor can be used at the output.

Two things worries me at the moment:

1st, the DAC's outputting a 0-2.5V sine wave, when the voltage output of the DAC drops, the inductance of the coil will cause reverse current feed back into the output pin of the DAC, I'm worrying about it will damage the DAC component....

2nd, for a coil of 24 Ohm, how can I keep the current low while not keep most power on the coil instead of the resistor I put on....

[taliesin]

Hi everyone!

My coil has a resistance of 24 Ohm and a inductance of 13mH, and I'm using DAC8581 from Texas Instrument to drive the coil, the datasheet suggests:

The DAC8581 uses a high-performance, rail-to-rail output buffer capable of driving a 600-Ω, 200-pF load with fast 0.65-μs settling. The buffer has exceptional noise performance (20 nV/√Hz) and fast slew rate (35 V/μs). The small-signal settling time is under 300 ns, allowing update rates up to 3 MSPS. Loads of 50 Ω or 75 Ω could be driven as long as output
current does not exceed ±25 mA continuously. Long cables, up to 1 nF in capacitance, can be driven without the use of external buffers. To aid stability under large capacitive loads (>1 nF), a small series resistor can be used at the output.

Two things worries me at the moment:

1st, the DAC's outputting a 0-2.5V sine wave, when the voltage output of the DAC drops, the inductance of the coil will cause reverse current feed back into the output pin of the DAC, I'm worrying about it will damage the DAC component....

2nd, for a coil of 24 Ohm, how can I keep the current low while not keep most power on the coil instead of the resistor I put on....

24 ohm ??

[Mizha Mekka]

Sorry, i was trying to say a coil with a resitance of 24 Ohm and a inductance of 13.4mH.....

[Carl-NC]

Is this for a VLF design? What frequency?

[Mizha Mekka]

The frequency is around 500-1000Hz.

[Rumcajs]

DAC8581

Hi,

the good output voltage is must be between cca. 8-15V, otherwise You getting from receiver coil only noise-level inputs.
(... but if You have an extra-hyper-super-selectivity receiver circuit from the NASA...)

Driving it with power-circuit, to eliminate low-voltage output and feedback-transient problems.

Here is one driver circuit both .bmp and .pdf format:


Brg

Rumcajs

[Altra]

This would work also, 16 - 60V supply, adjustable gain, 3amp

http://www.national.com/mpf/LM/LM675.html#Overview

[Carl-NC]

At 500Hz, your coil has a |Z| of 48 ohms, so you will be limited to the ±25mA max current, which is ±1.2v. Whether the DAC can drive such an inductive load is the question, and the only way to find out is try it. At the worst it will be unstable and ring or oscillate, but is unlikely to damage anything. My guess is that it will work OK for low frequency sinusoids.

- Carl

[Mizha Mekka]

Thanks, Carl. What I did was a RC passive filter at the output of DAC and before the input of Op-Amp, however, the waveform distorted badly --- in terms of both amplitude and phase, however, individually conneting the ampilifier to the coil seems to work, but I'm still not getting filtered signal... I wonder why is that...

[porkluvr]

Instead of using a passive filter followed by an opamp,
why not use an active bandpass filter to drive the coil?

You can get FilterPro to do the design work. Quick and easy (and free).

http://focus.ti.com/docs/toolsw/fold...filterpro.html


edit: Or, design an active lowpass filter if you desire frequency agility.