Datasheet LTC2382-16 (Analog Devices) - 10

LTC2382-16
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 LTC2382-16. 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 LTC2382-16 the current spike the ADC inputs draw 2.5V 1.8V TO 5V . is not acquiring the input, the analog inputs draw only a small leakage current. ANALOG INPUT 10µF 0.1µF 0V TO 2.5V
Input Filtering
The noise and distortion of the buffer amplifier and signal VDD OVDD CHAIN 011...111 IN+ RDL/SDI LT6350 source must be considered since they add to the ADC noise SDO LTC2383-16 011...110 BIPOLAR 3300pF ZERO and distortion. Noisy input signals should be filtered prior SCK IN– BUSY SAMPLE CLOCK 15Ω CNV 000...001 to the buffer amplifier input with an appropriate filter to REF GND 2.5V 000...000 SINGLE-ENDED- minimize noise. The simple 1- 23816 TA01 20µF pole RC lowpass filter (LPF1) TO-DIFFERENTIAL 111...111 DRIVER shown in Figure 4 is sufficient for many applications. 111...110 Another filter network consisting of LPF2 and the 100Ω series 100...001 FSR = +FS – –FS input resistors should be used between the buffer and ADC OUTPUT CODE (TWO’S COMPLEMENT) 100...000 1LSB = FSR/65536 inputs to both minimize the noise contribution of the buffer and to help minimize disturbances reflected into the buffer –FSR/2 –1 0V 1 FSR/2 – 1LSB LSB LSB from sampling transients. Long RC time constants at the INPUT VOLTAGE (V) 238216 F02 analog inputs will slow down the settling of the analog inputs.
Figure 2. LTC2382-16 Transfer Function
Therefore, LPF2 requires a wider bandwidth than LPF1. A buffer amplifier with a low noise density must be selected to REF minimize degradation of the SNR. With the 482kHz lowpass filter shown in Figure 4, the LT6350 provides the full data R C ON IN IN+ sheet performance of the LTC2382-16. High quality capacitors and resistors should be used in the BIAS RC filters since these components can add distortion. NPO REF VOLTAGE and silver mica type dielectric capacitors have excellent R C ON IN IN– linearity. Carbon surface mount resistors can generate distortion from self heating and from damage that may occur during soldering. Metal film surface mount resistors 238216 F03 are much less susceptible to both problems.
Figure 3. The Equivalent Circuit for the Differential Analog Input of the LTC2382-16
LPF2 SINGLE-ENDED- 50Ω 100Ω INPUT SIGNAL LPF1 IN+
INPUT DRIVE CIRCUITS
500Ω 3300pF LT6350 LTC2382-16 6600pF A low impedance source can directly drive the high im- IN– 50Ω 100Ω pedance inputs of the LTC2382-16 without gain error. A SINGLE-ENDED- BW = 482kHz 238216 F04 TO-DIFFERENTIAL high impedance source should be buffered to minimize BW = 48kHz DRIVER settling time during acquisition and to optimize the dis-
Figure 4. Input Signal Chain
tortion performance of the ADC. Minimizing settling time 238216fa 10 For more information www.linear.com/LTC2382-16