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I've designed an analogue front end for EMG signals that I want to turn into a PCB. However currently it runs off of two 9V batteries which makes the end product quite large and heavy. I was wondering how I would go about changing my design (or components) to a single battery supply?

Thanks!

Here's a picture of the the design(I apologise about splitting the circuit picture it's too big to display as one!) I use TL072's for op amps and an ina129 for the instrumentation amp:

enter image description here enter image description here

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Do you actually need 18V differential between positive and negative rail?

If the capacity of a 9V battery is sufficient, and you need 18V, then you could use a boost converter to generate 18V relative to battery negative, and then use the battery + terminal as "ground" and the - terminal as "-9V" and the boost output as "+9V."

If you can get away with a smaller voltage differential, then you can use one positive and one negative voltage regulator to generate positive and negative output voltages from a single-ended voltage.

You can also use a negative boost converter to get +9V and 0V from battery, and -9V from the negative DC DC converter.

If your system is very low-current/low-power, then finding the right components and possibly designing the right switching converters, with low quiescent current, will likely require some hard work (but is totally possible.)

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Observations:

  1. Power source / power requirement:

    If the device operates on 2 9 Volt batteries, then the power requirement is probably not very high: Typical 9 Volt batteries work best at low current anyway. Thus, switching to AA cells or even AAA cells is an option.
    In addition, lithium AA cells are relatively less expensive than their 9 Volt counterparts, with more energy per weight than Zinc cells.
    This leads to a suggestion to use just 2 or 4 AA cells, preferably Lithium chemistry, and a boost regulator, to obtain the voltages actually required in the device.

  2. Rail voltage reduction:

    It is worth examining whether +/- 9 Volt rails are really required.
    In the input stage of the design, signal voltage is pretty low as indicated in the schematic, so signal clipping to supply rails is probably not an issue.
    For subsequent amplification / filter stages, if the op-amps are selected to be single-supply, rail-to-rail input/output (RRIO) type, this eliminates the need for a dual supply rail.
    At the same time these op-amps may permit reducing the supply rail voltage, due to lower head-room needed by the op-amps, while retaining output signal voltage traversal as required by the ADC.

  3. Dual rail power (+/-9 Volts):

    Even if suitable single supply op-amps are not found, by using a virtual ground such as one of the several other options noted in this article, a dual-supply op-amp can be used on a single-rail design.
    This simplifies the design further, as a single boost regulator, delivering say 5 Volts or whatever the ADC uses for supply, will allow the device to operate, possibly even with merely two AA Lithium cells.

Thus, the answer:

  • Use 2x AA Lithium cells instead of a 9 Volt battery. This could actually work even with a single AA Lithium cell, note the minimum input voltage below.
  • Boost the supply using a suitable boost regulator, such as the very inexpensive and simple to use Holtek HT7750 boost (100 mA 5 Volt, supports input supply voltage down to 0.9 Volts)
  • Eliminate the dual rail requirement using a virtual ground circuit or a rail splitter IC (TLE2426)
  • Eliminate op-amp headroom using an RRIO single-supply op-amp such as AD8630 (Zero-Drift, Single-Supply, Rail-to-Rail Input/Output, auto-zeroing, ultra low noise, quad op amp)
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  • \$\begingroup\$ +1, but it bothers me a bit when people mention "single supply op-amps" as this implies they are some kind of different op-amp, and the ordinary kind can't be used on a single supply. This isn't really true: single supply op-amps are just more suited to single-supply operation in that they can swing their outputs closer to the rails and have a higher common-mode input range. Still, for amplification of small signals that don't even begin to approach the rails, I've used a TL072 on a single supply many times, with satisfactory results. \$\endgroup\$ – Phil Frost Jan 17 '14 at 13:16
  • \$\begingroup\$ @PhilFrost Oh I agree. It's just simpler to search for parts by keywords, and pretty much get all that you said in digest form in the search results :-) ... Incidentally, IIRC there were some op-amps, probably obsolete now, which actually had a ground pin, a +V pin and a -V pin - the true dinosaur dual-supply op amp. \$\endgroup\$ – Anindo Ghosh Jan 17 '14 at 13:38
  • \$\begingroup\$ Thanks for the all the comments! I'll redesign it with these considerations \$\endgroup\$ – alto125 Jan 17 '14 at 17:24
  • \$\begingroup\$ @AnindoGhosh Just to clarify, I was looking at the rail splitter/quad core op amps connection and I was wondering if the rail splitter gives me a virtual ground at half the input voltage, is the V- that I input into the quad core just the negative voltage form the AA batteries or the virtual ground? Newbie question but thought I'd ask! \$\endgroup\$ – alto125 Jan 19 '14 at 2:30
  • \$\begingroup\$ @alto125 The virtual-ground is used, if you need a reference that is midway between the supply rails, anywhere in the circuit. The supply rails themselves are the two terminals of your battery or your boost regulator output. \$\endgroup\$ – Anindo Ghosh Jan 19 '14 at 5:00

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