# D/A converter help plze

Discussion in 'General Electronics Chat' started by kaznov, Apr 18, 2012.

1. ### kaznov Thread Starter Member

Mar 10, 2012
30
0
the attached circuit is the 4-bit resistor ladder D/A with digital input of 0001 . for the simplcity we assume all values of R is the same . I didnt really get how the analoug output voltage came to be -(1/16)Vs . I did combine 2R with 2R which they located in the farthest right so they are in parellel and I know that the gain (V out/V in ) for this inverting circuit is -R(feedback) / R but I still didnt get Vout = - 1/16 Vs

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2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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A simple approach is to assume a current Io flows via the 2R resistor via the bo active switch and into the Op-Amp feedback resistor R.

This gives a Vout=-Io*R

The Op-Amp feedback will force the (switched) bo switch node to ground potential. Hence if Io is flowing via the 2R resistor & bo switch then the rightmost 2R resistor in the R-2R ladder network will also have an equal current Io flowing to the rightmost ground terminal.

The potential at the top-line rightmost ladder network node will then be Io*2R volts.

The rightmost R resistor in the ladder network will therefore be carrying a current of (Io+Io)=2*Io amps. The voltage drop across this resistor will be 2*Io*R volts, meaning the potential at the next left node of the ladder network will be 4*Io*R volts.

The current in the 2R resistor at the grounded (un-switched) b1 switch position will then be 2*Io amps. This means the current in the next left R resistor in the R-2R will be 4*Io amps. This will give a potential at the next left node in the ladder network of 8*Io*R volts.

The current in the 2R resistor at the grounded b2 switch position will then be 4*Io amps. This means the current in the next left R resistor in the ladder network will be 8*Io amps, giving a potential at the next left-hand node in the R-2R network of 16*Io*R volts. This will in fact be the same potential as Vs.

That is, Vs=16*Io*R volts.

One should note in passing that the (un-switched) grounded b3 switch condition has no bearing on the output voltage.

We know from the initial assumptions above that Vout=-Io*R

In addition knowing Vs=16*Io*R we may eliminate the unknown Io and conclude that Vout=-Vs/16 volts

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Posting a rough image of the annotated schematic to 'match' with the previous verbose post.

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4. ### kaznov Thread Starter Member

Mar 10, 2012
30
0
thanks alot guys that helpes a lot