SAR ADC Input Types 5V 10V IN SAR ADC IN 0V SAR ADC –10V GND GND Figure 1a. Single-Ended Unipolar Figure 1b. Single-Ended True Bipolar Single-Ended Inputs An ADC with single-ended inputs digitizes the analog input voltage relative to ground. Single-ended inputs simplify ADC driver requirements, reduce complexity and lower power dissipation in the signal chain. Single-ended inputs can either be unipolar or bipolar, where the analog input on a single-ended unipolar ADC swings only above GND (0V to VFS, where VFS is the full-scale input voltage that is determined by a reference voltage) (Figure 1a) and the analog input on a single-ended bipolar ADC also called true bipolar, swings above or below GND (±VFS) (Figure 1b). 5V 0V 5V IN+ SAR ADC IN – 0V 2.5V GND Figure 2a. Pseudo-Differential Unipolar 10V IN+ IN+ SAR ADC IN– –10V SAR ADC IN– GND Figure 2b. Pseudo-Differential Bipolar GND Figure 2c. Pseudo-Differential True Bipolar Pseudo-Differential Inputs An ADC with pseudo-differential inputs digitizes the differential analog input voltage (IN+ – IN–) over a limited range. The IN+ input has the actual analog input signal, while the IN– input has a restricted range. A pseudo-differential unipolar ADC digitizes the differential analog input voltage (IN+ – IN–) over a span of 0V to VFS. In this range, a single-ended unipolar input signal, driven on the IN+ pin, is measured with respect to the signal ground reference level, driven on the IN– pin. The IN+ pin is allowed to swing from GND to VFS , while the IN– pin is restricted to around GND ± 100mV (Figure 2a). A pseudo-differential bipolar ADC digitizes the differential analog input voltage (IN+ – IN–) over a span of ±VFS /2. In this range, a single-ended bipolar input signal, driven on the IN+ pin, is measured with respect to the signal mid-scale reference level, driven on the IN– pin. The IN+ pin is allowed to swing from GND to VFS, while the IN– pin is restricted to around VFS /2 ± 100mV (Figure 2b). A pseudo-differential true bipolar ADC digitizes the differential analog input voltage (IN+ – IN–) over a span of ±VFS . In this range, a true bipolar input signal, driven on the IN+ pin, is measured with respect to the signal ground reference level, driven on the IN– pin. The IN+ pin is allowed to swing above or below GND to ±VFS , while the IN– pin is restricted to around GND ± 100mV (Figure 2c). Pseudo-differential inputs help separate signal ground from the ADC ground, allowing small common-mode voltages to be cancelled. They also allow single-ended input signals that are referenced to ADC ground. Pseudo-differential ADCs are ideal for applications that require DC common-mode voltage rejection, for single-ended input signals and for applications that do not want the complexity of differential drivers. Pseudo-differential inputs simplify the ADC driver requirement, reduce complexity and lower power dissipation in the signal chain. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 5V 10V IN+ 0V IN+ –10V SAR ADC 5V IN– SAR ADC IN– 10V GND 0V GND –10V Figure 3a. Fully Differential Figure 3b. Fully Differential True Bipolar Fully Differential Inputs An ADC with fully-differential inputs digitizes the differential analog input voltage (IN+ – IN–) over a span of ±VFS. In this range, the IN+ and IN– pins should be driven 180º out-of-phase with respect to each other, centered on a fixed common mode voltage, for example, VREF /2 ±50mV. In most fully-differential ADCs, both the IN+ and IN– pins are allowed to swing from GND to VFS (Figure 3a), while in fully-differential true bipolar ADCs, both the IN+ and IN– pins are allowed to swing above or below GND to ±VFS (Figure 3b). Fully-differential inputs offer wider dynamic range and better SNR performance over single-ended or pseudo-differential inputs. Fully differential ADCs are ideal for applications that require the highest performance. IN+, IN– ARBITRARY IN+, IN– DIFFERENTIAL 5V 5V 0V 0V ARBITRARY IN+ DIFFERENTIAL 5V 5V –5V –5V IN+ SAR ADC SAR ADC BIPOLAR UNIPOLAR 5V 5V 0V 0V IN BIPOLAR – GND UNIPOLAR 5V 5V –5V 0V Figure 4a. Differential with Wide Input Common Mode IN – GND Figure 4b. Differential True Bipolar Differential Inputs with Wide Input Common Mode An ADC with differential inputs digitizes the voltage difference between the IN+ and IN– pins while supporting a wide common mode input range. The analog input signals on IN+ and IN– can have an arbitrary relationship to each other. In most differential ADCs, both IN+ and IN– remain between GND and VFS (Figure 4a), while in differential true bipolar ADCs, both the IN+ and IN– pins are allowed to swing above or below GND to ±VFS (Figure 4b). Differential inputs are ideal for applications that require a wide dynamic range with high common mode rejection. Being one of the most flexible ADC input types, an ADC with differential inputs can also digitize other types of analog input signals such as single-ended unipolar, pseudo-differential unipolar/bipolar and fully-differential. Input Types Single-Ended Pseudo-Differential LTC1865, LTC2314, LTC2315, LTC2360, LTC2361, LTC2362, LTC2365, LTC2366 Single-Ended True Bipolar LTC1400, LTC1404, LTC1605, LTC1606, LTC1609 Pseudo-Differential Unipolar LTC1864, LTC2305, LTC2306, LTC2308, LTC2309, LTC2364, LTC2367, LTC2368, LTC2369, LTC2370, LTC2389, LTC2372, LTC2373 Pseudo-Differential Bipolar LTC2305, LTC2306, LTC2308, LTC2309, LTC2389, LTC2372, LTC2373 Pseudo-Differential True Bipolar LTC1414, LTC1419, LTC1854, LTC1855, LTC1856, LTC1857, LTC1858, LTC1859, LTC2328, LTC2327, LTC2326 Fully Differential LTC2376, LTC2377, LTC2378, LTC2379, LTC2380, LTC2383, LTC2389, LTC2393, LTC2372, LTC2373 Fully Differential True Bipolar LTC2338, LTC2337, LTC2336 Differential LTC1403, LTC1407, LTC1408, LTC2351, LTC2355, LTC2356, LTC2323, LTC2321, LTC2348 Differential True Bipolar LTC1604, LTC1608, LTC2348 Fully Differential Differential with Wide Input Common Mode Linear Technology SAR ADCs Single-Ended Unipolar www.linear.com/SARinputtypes n 1-800-4-LINEAR 0615B