2PB_sarinputtypesfb.pdf

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.
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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
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