# Output impedance of a voltage divider with DC bias

I made a circuit simulation of a 200VAC to 0-2VDC for ADC conversion via voltage divider and DC bias with a third resistor connected to a 3.3V supply. What is the output impedance of the divider and how do I calculate it?

• Are you sure your 1M can withstand 240V rms or 500V transients or 3kV? – Tony Stewart EE75 Mar 18 at 17:59
• Your ratio is 399:1 and biased at 1.0V. To make better measurements you need a rectified precision peak & hold and also an average detector then you can do 1 sample per cycle. and convert AVG to RMS and also measure transient disturbances. – Tony Stewart EE75 Mar 18 at 18:02
• Also note that the circuit is not isolated and thus the whole circuit must be considered live and it should have no connections to any other circuitry or human touchable parts. – Justme Mar 18 at 18:22
• If line to line its is live , if Line to Netrual it is almost live with Neutral grounded but unprotected against CM lightning spikes. – Tony Stewart EE75 Mar 18 at 23:55

The output impedance is not affected by the DC bias.

Treat the DC supply as a short circuit.

The output impedance is all three resistors in parallel - 3.6k//8.3k//1meg.

The input resistance is then just the parallel impedance of the 8.3k and 3.6k resistors in series with the 1 megohm input resistor.

In practice, the input impedance would normally be considered to be just 1 megohm because the 8.3k//3.6k is significantly less than 1% of the input resistor and unless precision resistors are used will be in error by less than the tolerance of the resistors.

Similarly, for the output impedance, you can ignore the 1megohm input resistance as it will have a very minor effect.

• I'm a bit confused though, how come the ratio here is different? tinyurl.com/yh4nr8kh – BeB00 Mar 18 at 18:16
• @BeB00 - my error, I was just considering the impedance of the 8.3k//3.6k resistors. – Kevin White Mar 18 at 18:22
• Oh I see, I misunderstood your answer, you were saying that the DC bias itself had no effect, but the resistors for it did – BeB00 Mar 18 at 18:24
• @BeB00 compare this tinyurl.com/yey7282z – Tony Stewart EE75 Mar 19 at 0:26