 to the number of levels which must be coded. For ter's dynamic range contains the input signal and gives the corre- example, for the case of a 4-bit flash converter, capable of coding sponding binary code output. voltages in the range () to 1 V, 16 comparators are needed, connected The main feature of the flash converter is its extremely rapid to a resistor chain containing 16 equal resistors, connected across a conversion time. They are often used for video ADC's. or any other application which requires a sample rate in excess of 1MHz. The +1.0V penalty for the speed is the rapid increase in complexity, and therefore power consumption and cost, as the desired number of bits increases. Indeed, it is rare to find a flash converter operating to more than 8-bit accuracy because of the cost of an array of more than 256 comparators. Variation of input offset voltage between adjacent comparators can set a further limit to ultimate resolution. However, analogue and digital pipelining can be combined to im- plement two stage flash conversion, giving accuracy up to twice the number of bits with the same sample rate, and only twice the conversion time. The above converters encode voltages between ground and some upper limit. It is more usual to want to convert voltages of both polarities. This can be achieved either by 3 bit binary output 1. The addition of a voltage equal to half the upper limit, with two's complement coding of the signal then easily achieved by inversion of the resulting MSB. 2. The use of a precision full wave detector, which results in an output signal of fixed polarity, together with information about whether the signal is positive or negative. The latter scheme results naturally in the production of a 'sign- plus-magnitude' code. 19.4.3 The decoder Figure 19.17 A 3 bit flash converter After the digitally encoded signal has been transmitted, it is often necessary to reconstruct an analogue signal at the receiver. Conver- Pulse code modulation 19/15 sion from digital to analogue is generally easier to