A flash converter is an A/D design where the input voltage is compared to several reference voltages in descending priority. This converter can be summed up in terms of three components: a reference voltage connected to a cascading series of resistors, a series of op-amps, and a priority encoder, as shown below.

The pictured flash converter has a digital resolution of three bits. The analog input goes into the positive terminal of each op-amp. Vref+ and Vref- are connected to a resistor network that evenly divides the differential voltage (VLSB = (Vref+ - Vref-) / 8, where n = 3 since there are eight divisions in the resistor network) at each node. Each of these nodes go into the negative terminal of its adjacent op-amp. Notice that the topmost op-amp compares the analog input to Vref- + 7 * VLSB, then the one below it compares the analog input to Vref- + 6 * VLSB, then the next one compares it to Vref- + 5 * VLSB, and so on and so forth. If the analog input is greater than a particular reference, then the op-amps will produce a logic high (1). If not, the logic output will be low. This is where the priority encoder comes into play: if the topmost op-amp outputs a 1 (that is, D7 = 1), then so will all the op-amps below it. The priority encoder gives highest priority to the topmost pin, so if the op-amp feeding that pin sends a 1 to the encoder, then the output must be 111. Similarly, if the fourth op-amp from the bottom outputs a 1 (D4 = 1), with the ones above it outputting 0, then the third, second, and first op-amps will also be 1. The priority encoder gives priority to the fourth op-amp, thus the output would be 100.

For reference, the following behavior describes how the pictured flash converter works: 

The flash converter is a simple and effective design that makes conversions at a very fast rate. The largest drawback to this A/D, though, is that it is expensive to build. For a digital resolution of n, a flash converter will need 2^n - 1 resistors, 2^n - 1 op-amps, and a 2^n-to-n bit priority encoder. So as the digital resolution of the converter increases, so do the amount of components needed. The flash converter design, however, is one of several A/D implementations. Another possible A/D design is known as the successive approximation register.