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MAX147AEAP Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX147AEAP Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 24 page +2.7V, Low-Power, 8-Channel, Serial 12-Bit ADCs _______________________________________________________________________________________ 9 _______________Detailed Description The MAX146/MAX147 analog-to-digital converters (ADCs) use a successive-approximation conversion technique and input track/hold (T/H) circuitry to convert an analog signal to a 12-bit digital output. A flexible ser- ial interface provides easy interface to microprocessors (µPs). Figure 3 is a block diagram of the MAX146/ MAX147. Pseudo-Differential Input The sampling architecture of the ADC’s analog com- parator is illustrated in the equivalent input circuit (Figure 4). In single-ended mode, IN+ is internally switched to CH0–CH7, and IN- is switched to COM. In differential mode, IN+ and IN- are selected from the fol- lowing pairs: CH0/CH1, CH2/CH3, CH4/CH5, and CH6/CH7. Configure the channels with Tables 2 and 3. In differential mode, IN- and IN+ are internally switched to either of the analog inputs. This configuration is pseudo-differential to the effect that only the signal at IN+ is sampled. The return side (IN-) must remain sta- ble within ±0.5LSB (±0.1LSB for best results) with respect to AGND during a conversion. To accomplish this, connect a 0.1µF capacitor from IN- (the selected analog input) to AGND. During the acquisition interval, the channel selected as the positive input (IN+) charges capacitor CHOLD. The acquisition interval spans three SCLK cycles and ends on the falling SCLK edge after the last bit of the input control word has been entered. At the end of the acqui- sition interval, the T/H switch opens, retaining charge on CHOLD as a sample of the signal at IN+. The conversion interval begins with the input multiplex- er switching CHOLD from the positive input (IN+) to the negative input (IN-). In single-ended mode, IN- is sim- ply COM. This unbalances node ZERO at the compara- tor’s input. The capacitive DAC adjusts during the remainder of the conversion cycle to restore node ZERO to 0V within the limits of 12-bit resolution. This action is equivalent to transferring a 16pF x [(VIN+) - (VIN-)] charge from CHOLD to the binary-weighted capacitive DAC, which in turn forms a digital represen- tation of the analog input signal. Track/Hold The T/H enters its tracking mode on the falling clock edge after the fifth bit of the 8-bit control word has been shifted in. It enters its hold mode on the falling clock edge after the eighth bit of the control word has been shifted in. If the converter is set up for single-ended inputs, IN- is connected to COM, and the converter samples the “+” input. If the converter is set up for dif- ferential inputs, IN- connects to the “-” input, and the difference of |IN+ - IN-| is sampled. At the end of the conversion, the positive input connects back to IN+, and CHOLD charges to the input signal. INPUT SHIFT REGISTER CONTROL LOGIC INT CLOCK OUTPUT SHIFT REGISTER +1.21V REFERENCE (MAX146) T/H ANALOG INPUT MUX 12-BIT SAR ADC IN DOUT SSTRB VDD DGND AGND SCLK DIN COM REFADJ VREF OUT REF CLOCK +2.500V 20k *A ≈ 2.00 (MAX147) 10 11 12 9 15 16 17 18 19 CH6 7 CH7 8 CH4 5 CH5 6 CH1 2 CH2 3 CH3 4 CH0 1 MAX146 MAX147 CS SHDN 20 14 13 ≈ 2.06* A Figure 3. Block Diagram CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM CSWITCH TRACK T/H SWITCH RIN 9k CHOLD HOLD 12-BIT CAPACITIVE DAC VREF ZERO COMPARATOR – + 16pF SINGLE-ENDED MODE: IN+ = CH0–CH7, IN- = COM. DIFFERENTIAL MODE: IN+ AND IN- SELECTED FROM PAIRS OF CH0/CH1, CH2/CH3, CH4/CH5, AND CH6/CH7. AT THE SAMPLING INSTANT, THE MUX INPUT SWITCHES FROM THE SELECTED IN+ CHANNEL TO THE SELECTED IN- CHANNEL. INPUT MUX Figure 4. Equivalent Input Circuit |
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