# Finding ground in a battery powered circuit?

I don't know if this is an obvious question. I'm powering my circuit (that's used to amplify an EKG signal) by a ±5V battery supply, the amplifier I'm using the ina129 needs a voltage reference that's midway between my voltage supplies.

I considered using a simple voltage divider circuit but found that it becomes unbalanced. Could I use something like the Texas Instruments virtual ground splitter (tle2426) or does it only work for positive voltage supplies?

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If you have +5V and -5V then half way between supply voltages is 0V i.e. the common connection of the two batteries - this should be regarded as your "ground" (or 0V) for your circuit. If you read what TI say about the TLE2426 you'll possibly agree: -

In signal-conditioning applications utilizing a single power source, a reference voltage equal to one-half the supply voltage is required for termination of all analog signal grounds. Texas Instruments presents a precision virtual ground whose output voltage is always equal to one-half the input voltage, the TLE2426 “rail splitter.”

You have two power sources namely +5V and -5V and these are likely to have a third wire which you have available that does exactly what the TI chip sets out to do.

Why did the INA129 become unbalanced: -

Using a resistor divider to feed the REF input means that you are altering the 40k resistance at pin 5 and this will unbalance the ratios of resistors and lead to common-mode signal problems.

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You should really buffer that reference voltage. The datasheet probably says so, and it generally doesn't cost much to do so. – Matt Young Feb 17 '14 at 13:14
Thanks for the fast reply! My circuit doesn't have two batteries it's powered from a 3.7v battery which I'm boosting up to 5V and also inverting to get -5V so I don't have a ground wire initially, would this make any difference? – alto125 Feb 18 '14 at 6:29
Ah... If you have a 0V that is centred between +5 and -5, that should be your choice of 0V. – Andy aka Feb 18 '14 at 8:02

Voltages are relative things. In practice, it's best to always think of them in terms of a reference voltage which you have available (and which, consequently, is usually the ground)

If you already have +/-5V, it's likely that you have a ground present (0V). If you're using a single 10V battery and just treating them as +/-5, then it really is only a difference in bookkeeping on your part. The reason I'm saying this, is because

does it only work for positive voltage supplies?

suggests that you seem slightly confused with regards to ground. If you have no ground, and you give the IC +/-5V, this is practically identical to giving it 0/10V. Even if you did have a ground somewhere, and still gave the IC +/-5V, it is no different from giving it 0/10V unless you have also given it GND through a pin which it treats as grounds, since as far as the IC is concerned, the only supplies of consequence are it's 0/10V inputs. You should, though, make sure that every pin of the said IC is within the limits specified within the datasheet. Beyond that, positive and negative in the context of a single voltage node is only relevant in the context of a specific ground or other 'reference node', and isn't a god-given distinction.

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