From: "Tom Faloon"
Subject: Re: LTC1298 voltage reference question
Date: Tue, 24 Sep 2002 13:14:14 +0100
NNTP-Posting-Date: Tue, 24 Sep 2002 12:52:08 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
I would not normally consider using a 7805 regulator as a reference for a 12
bit analogue to digital converter. There are a great many voltage references
available which have been designed for this type of application, and which
would be infinitely more suitable.
However, you didn't say anything about your application. If you just want
something cheap, and don't care too much about resolution then you could use
a 7805. The current demand is tiny, you maybe use a 78L05, which is a much
If you do use a 7805 / 78L05 consider the following.
DC Output Voltage
The output voltage will be somewhere within limits 4.75 to 5.25 Volts. This
is a pretty wide spec. Probably much better than this, but the data sheet
won't tell you much more.
Output voltage will vary with temperature at approximatelly 1.1mV/Deg C
(Probably not important if you can live with the wide Voltage spec.)
Ripple + Noise.
The output should be fairly quiet. The 7805 will reject any ripple on the 12
V supply by more than 60dB, and add very little noise of it's own.
If there is a lot of high frequency noise on the 12V supply, you could fit
an RC filter at the regulator input. Anyway, I don't think this will be a
The LTC1296 requires only 140uA maximum at it's reference input, but the
9805 needs at least a 5mA load to maintaim proper regulation, so fit a 1K,
or smaller, resistive load at the regulator output.
Fit a 0.1uF ceramic capacitor at the regulator output.
V8TR4 wrote in message
> I'm using a LTC1298 ADC in a project. The LTC uses the VCC (5vdc) as its
> reference. I am thinking about using a 7805 off of 12v line to to power
> with nothing else hanging on it. I had noticed that I was getting a slight
> amount of change in reference when I ran it off the 5vdc bus. The data
> says I can expect 40-200mv change. Since the LTC is only thing on the 7805
> thinking it should be pretty stable.
> Any opinions on doing this?