Datasheet LTC2381-16 (Analog Devices) - 10

LTC2381-16
APPLICATIONS INFORMATION
shown in Figure 3. The diodes at the input provide ESD is important even for DC inputs, because the ADC inputs protection. In the acquisition phase, each input sees ap- draw a current spike when entering acquisition. proximately 45pF (CIN) from the sampling CDAC in series For best performance, a buffer amplifier should be used with 40Ω (RON) from the on-resistance of the sampling to drive the analog inputs of the LTC2381-16. The ampli- switch. Any unwanted signal that is common to both in- fier provides low output impedance which produces fast puts will be reduced by the common mode rejection of the settling of the analog signal during the acquisition phase. ADC. The inputs draw a current spike while charging the It also provides isolation between the signal source and CIN capacitors during acquisition. When the LTC2381-16 the current spike the ADC inputs draw. is not acquiring the input, the analog inputs draw only a 2.5V 1.8V TO 5V small leakage current.
Input Filtering
ANALOG INPUT 10µF 0.1µF 0V TO 2.5V The noise and distortion of the buffer amplifier and signal 011...111 source must be considered since they add to the ADC noise VDD OVDD CHAIN IN+ RDL/SDI 011...110 BIPOLAR ZERO LT6350 and distortion. Noisy input signals should be filtered prior SDO 3300pF LTC2383-16 SCK 000...001 to the buffer amplifier input with an appropriate filter to IN– BUSY SAMPLE CLOCK 15Ω CNV 000...000 minimize noise. The simple REF 1-pole RC lowpass filter (L GND PF1) 2.5V 23816 TA01 111...111 SINGLE-ENDED- shown in Figure 4 is sufficient for many applications. TO-DIFFERENTIAL 20µF 111...110 DRIVER Another filter network consisting of LPF2 and the 100Ω 100...001 FSR = +FS – –FS series input resistors should be used between the buffer OUTPUT CODE (TWO’S COMPLEMENT) 100...000 1LSB = FSR/65536 and ADC inputs to both minimize the noise contribution of the buffer and to help minimize disturbances reflected –FSR/2 –1 0V 1 FSR/2 – 1LSB LSB LSB into the buffer from sampling transients. Long RC time INPUT VOLTAGE (V) 238116 F02 constants at the analog inputs will slow down the settling
Figure 2. LTC2381-16 Transfer Function
of the analog inputs. Therefore, LPF2 requires a wider bandwidth than LPF1. A buffer amplifier with a low noise REF density must be selected to minimize degradation of the SNR. With the 482kHz lowpass filter shown in Figure 4, R C ON IN IN+ the LT6350 provides the full data sheet performance of the LTC2381-16. BIAS High quality capacitors and resistors should be used in the REF VOLTAGE RC filters since these components can add distortion. NPO R C ON IN IN– and silver mica type dielectric capacitors have excellent linearity. Carbon surface mount resistors can generate distortion from self heating and from damage that may 238116 F03 occur during soldering. Metal film surface mount resistors
Figure 3. The Equivalent Circuit for the Differential Analog Input of the LTC2381-16
are much less susceptible to both problems. LPF2 SINGLE-ENDED- 50Ω 100Ω
INPUT DRIVE CIRCUITS
INPUT SIGNAL LPF1 IN+ 500Ω 3300pF LT6350 LTC2381-16 A low impedance source can directly drive the high im- 6600pF IN– pedance inputs of the LTC2381-16 without gain error. A 50Ω 100Ω high impedance source should be buffered to minimize SINGLE-ENDED- BW = 482kHz 238116 F04 BW = 48kHz TO-DIFFERENTIAL settling time during acquisition and to optimize the dis- DRIVER tortion performance of the ADC. Minimizing settling time
Figure 4. Input Signal Chain
238116fa 10 For more information www.linear.com/LTC2381-16