Datasheet LTC2378-20 (Analog Devices) - 10

LTC2378-20
applicaTions inForMaTion
tling of the analog signal during the acquisition phase. It 011...111 also provides isolation between the signal source and the 011...110 BIPOLAR ZERO ADC input currents. 000...001 000...000
Noise and Distortion
111...111 The noise and distortion of the buffer amplifier and signal 111...110 source must be considered since they add to the ADC noise 100...001 and distortion. Noisy input signals should be filtered prior FSR = +FS – –FS OUTPUT CODE (TWO’S COMPLEMENT) 100...000 1LSB = FSR/1048576 ≈ 1ppm to the buffer amplifier input with an appropriate filter to minimize noise. The simple 1-pole RC lowpass filter (LPF1) –FSR/2 –1 0V 1 FSR/2 – 1LSB LSB LSB shown in Figure 4 is sufficient for many applications. INPUT VOLTAGE (V) 237820 F02
Figure 2. LTC2378-20 Transfer Function
LPF2 6800pF shown in Figure 3. The diodes at the input provide ESD SINGLE-ENDED- 10Ω INPUT SIGNAL LPF1 protection. In the acquisition phase, each input sees ap- IN+ 500Ω 3300pF proximately 45pF (C LTC2378-20 IN) from the sampling CDAC in series with 40Ω (R 6600pF IN– ON) from the on-resistance of the sampling 10Ω switch. Any unwanted signal that is common to both SINGLE-ENDED- 6800pF 237820 F04 inputs will be reduced by the common mode rejection of BW = 48kHz TO-DIFFERENTIAL DRIVER the ADC. The inputs draw a current spike while charging BW = 1.2MHz the C
Figure 4. Input Signal Chain
IN capacitors during acquisition. During conversion, the analog inputs draw only a small leakage current. A coupling filter network (LPF2) should be used between the buffer and ADC input to minimize disturbances reflected
INPUT DRIVE CIRCUITS
into the buffer from sampling transients. Long RC time A low impedance source can directly drive the high imped- constants at the analog inputs will slow down the settling ance inputs of the LTC2378-20 without gain error. A high of the analog inputs. Therefore, LPF2 typically requires a impedance source should be buffered to minimize settling wider bandwidth than LPF1. This filter also helps minimize time during acquisition and to optimize ADC linearity. For the noise contribution from the buffer. A buffer amplifier best performance, a buffer amplifier should be used to with a low noise density must be selected to minimize drive the analog inputs of the LTC2378-20. The amplifier degradation of the SNR. provides low output impedance, which produces fast set- High quality capacitors and resistors should be used in the RC filters since these components can add distortion. NPO REF C and silver mica type dielectric capacitors have excellent R IN ON 45pF 40Ω linearity. Carbon surface mount resistors can generate IN+ distortion from self heating and from damage that may occur during soldering. Metal film surface mount resistors BIAS are much less susceptible to both problems. REF VOLTAGE C R IN ON 45pF 40Ω
Input Currents
IN– One of the biggest challenges in coupling an amplifier to 237820 F03 the LTC2378-20 is in dealing with current spikes drawn
Figure 3. The Equivalent Circuit for the
by the ADC inputs at the start of each acquisition phase.
Differential Analog Input of the LTC2378-20
237820fb 10 For more information www.linear.com/LTC2378-20 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Converter Characteristics Dynamic Accuracy Reference Input Digital Inputs and Digital Outputs Power Requirements ADC Timing Characteristics Typical Performance Characteristics Pin Functions Functional Block Diagram Timing Diagram Applications Information Board Layout Package Description Revision History Typical Application Related Parts