15.01.2013 Views

U. Glaeser

U. Glaeser

U. Glaeser

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

FIGURE 11.14 Current-mode CMOS analog-to-quaternary converter block diagram.<br />

the first done on the input, then the resultant Q is the most-significant-digit (MSD) of the quaternaryvalued<br />

output. Having identified the quarter-of-full-scale within which the input lies, we eliminate from<br />

further consideration the other regions by subtracting the number of full quarters above which the input<br />

lies from the input signal. Equivalently, we may subtract Q · REF from 4IN and obtain four times this<br />

desired difference. This factor of 4 weight of the difference signal is necessary to keep the bit significance<br />

correct as we continue to process the signal. The quaternary signal Q controls the switch, which effectively<br />

subtracts Q · REF from 4IN. The output signal is thus<br />

After the appropriate quarter of full-scale that is now defined as the region-of-interest is identified, this<br />

new region-of-interest is then searched for the quarter within which the input signal lies. The signal OUT<br />

may be used as the input to another identical stage or the value of Q may be stored and the signal OUT<br />

fed back to the input of this circuit for continued processing. Each pass through the procedure yields<br />

another digit of one lower level of significance until we reach the final least-significant-digit (LSD)<br />

decision. Thus, the MSD is determined first and the LSD determined last. The procedure may be<br />

implemented with some memory, control logic, and a single cell that performs the operations in<br />

Fig. 11.14, feeding the output back to the input. Or the procedure can be implemented by a cascade of<br />

N cells, each using the same REF. In the next section, we describe the current-mode CMOS circuitry that<br />

implements this algorithmic analog-to-quaternary (A/Q) data converter function.<br />

The schematic of the current-mode CMOS algorithmic A/Q data converter circuit is shown in Fig. 11.15.<br />

The circuit operates as follows. For our initial discussion, assume that the bias current I BIAS is zero and<br />

PMOS transistor M 1 is not used. The analog input current I IN into diode-connected input NMOS<br />

transistor M 2 is reproduced and multiplied by a factor of 4, 2, 2, and 4 by NMOS current mirror transistors<br />

M 4, M 6, M 8, and M 20, respectively. The full scale reference current I REF is brought into diode connected<br />

PMOS transistor M 21 and reproduced and multiplied by a factor of 1, 1, 1.5, 1, 1, and 1 by PMOS current<br />

mirror transistors M 5, M 7, M 9, M 12, M 13, and M 14, respectively. Transistors M 4 and M 5 form a current<br />

comparator circuit that compares 4 I IN to the full-scale reference current I REF. Transistors M 6 and M 7<br />

form a current comparator circuit that compares 2I IN to the full-scale reference current I REF. Transistors<br />

© 2002 by CRC Press LLC<br />

OUT = 4IN – Q ⋅ REF.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!