If your normal voltage range of the signal is outside the range of the input of the MCP3002, you really need to do some sort of front-end signal conditioning. I.E. voltage divider into an opamp. See the note in section 4.1: "Ideally, the impedance of the signal source should be near zero. This is achievable with an operational amplifier such as the MCP601 which has a closed loop output impedance of tens of ohms. The adverse affects of higher source impedances are shown in Figure 4-2. " As cheap as fairly decent opamps are, I've taken to just putting a opamp wired for and appropriate amount of gain in front of pretty much all of my ADC's. I also tend to prefer to use them in inverting mode anymore since I really don't care if the slope out of the opamp is positive or negative - I can fix it in software. The calculation of input impedance (as seen by an external voltage input) into a inverting opamp is simpler (1M series resistor =3D 1M input impedance, more or less). If you look at figure 4-2, you'll note the lower the impedance the faster the ADC can be sampled. Ideally you'd keep it under 1K, which pretty much means an opamp unless your signal source can handle a divider which results in a 1K input impedance. On Wed, May 17, 2017 at 9:18 PM, James Cameron wrote: > I'm missing something terribly obvious, can anyone point it out? > > Assuming an MCP3002 powered from 3.3V, but an analog signal that may > swing up to X volts, what minimum series resistance is needed given > the maximum rating of Vdd + 0.6V? > > http://ww1.microchip.com/downloads/en/DeviceDoc/21294C.pdf > > See also Figure 4-1; how to protect the diodes between the CHx pin and > the Vdd pin. > > In general, I'd be happy with a quite large resistor. I'm just > curious as to what the minimum would be, because I couldn't find it in > the datasheet. > > -- > James Cameron > http://quozl.netrek.org/ > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 *Forrest Christian* *CEO**, PacketFlux Technologies, Inc.* Tel: 406-449-3345 | Address: 3577 Countryside Road, Helena, MT 59602 forrestc@imach.com | http://www.packetflux.com --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .