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LTC2309IUFXTRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC2309IUFXTRPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LTC2309 11 2309f APPLICATIONS INFORMATION When using a filter with a large CFILTER value (e.g. 1μF), the inputs do not completely settle and the capacitive input switching currents are averaged into a net DC current (IDC). In this case, the analog input can be mod- eled by an equivalent resistance (REQ = 1/(fSMPL • CIN)) in series with an ideal voltage source (VREFCOMP/2) as shown in Figure 3b. The magnitude of the DC current is then approximately IDC = (VIN – VREFCOMP/2)/REQ, which is roughly proportional to VIN. To prevent large DC drops across the resistor RFILTER, a filter with a small resistor and large capacitor should be chosen. When running at the maximum throughput rate of 14ksps, the input current equals 1.5μA at VIN = 4.096V, which amounts to a full-scale error of 0.5LSB when using a filter resistor (RFILTER) of 333Ω. Applications requiring lower sample rates can tolerate a larger filter resistor for the same amount of full-scale error. self heating and from damage that may occur during soldering. Metal film surface mount resistors are much less susceptible to both problems. Dynamic Performance Fast Fourier Transform (FFT) test techniques are used to test the ADC’s frequency response, distortion and noise at the rated throughput. By applying a low distortion sine wave and analyzing the digital output using an FFT algorithm, the ADC’s spectral content can be examined for frequencies outside the fundamental. Signal-to-Noise and Distortion Ratio (SINAD) The signal-to-noise and distortion ratio (SINAD) is the ratio between the RMS amplitude of the fundamental input frequency to the RMS amplitude of all other fre- quency components at the A/D output. The output is band-limited to frequencies from above DC and below half the sampling frequency. Figure 5 shows a typical SINAD of 73.3dB with a 14kHz sampling rate and a 1kHz input. An SNR of 73.4dB can be achieved with the LTC2309. VIN INPUT CH0-CH7 RON 100Ω CIN 55pF CFILTER RSOURCE 2309 F03a LTC2309 Figure 3a. Analog Input Equivalent Circuit VIN INPUT CH0-CH7 REQ 1/(fSMPL • CIN) VREFCOMP/2 CFILTER RFILTER IDC 2309 F03b LTC2309 + – Figure 3b. Analog Input Equivalent Circuit for Large Filter Capacitances Figures 4a and 4b show examples of input filtering for single-ended and differential inputs. For the single- ended case in Figure 4a, a 50Ω source resistor and a 2000pF capacitor to ground on the input will limit the input bandwidth to 1.6MHz. High quality capacitors and resistors should be used in the RC filter since these components can add distortion. NPO and silver mica type dielectric capacitors have excellent linearity. Carbon surface mount resistors can generate distortion from 2309 F04a CH0 COM LTC2309 REFCOMP 2000pF 0.1μF 10μF 50Ω ANALOG INPUT Figure 4a. Optional RC Input Filtering for Single-Ended Input 1000pF 2309 F04b CH0 CH1 LTC2309 REFCOMP 1000pF 1000pF 0.1μF 10μF 50Ω 50Ω DIFFERENTIAL ANALOG INPUTS Figure 4b. Optional RC Input Filtering for Differential Inputs |
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