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LTC2356-14 Datasheet(PDF) 11 Page - Linear Technology |
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LTC2356-14 Datasheet(HTML) 11 Page - Linear Technology |
11 / 24 page LTC2369-18 11 236918fa APPLICATIONS INFORMATION INPUT DRIVE CIRCUITS A low impedance source can directly drive the high im- pedance input of the LTC2369-18 without gain error. A high impedance source should be buffered to minimize settling time during acquisition and to optimize the dis- tortion performance of the ADC. Minimizing settling time is important even for DC inputs, because the ADC input draws a current spike when entering acquisition. For best performance, a buffer amplifier should be used to drive the analog input of the LTC2369-18. The ampli- fier provides low output impedance, which produces fast settling of the analog signal during the acquisition phase. It also provides isolation between the signal source and the current spike the ADC input draws. Input Filtering The noise and distortion of the buffer amplifier and signal source must be considered since they add to the ADC noise and distortion. Noisy input signals should be filtered prior to the buffer amplifier input with an appropriate filter to minimize noise. The simple 1-pole RC lowpass filter (LPF1) shown in Figure 4 is sufficient for many applications. High quality capacitors and resistors should be used in the RC filters since these components can add distortion. NPO and silver mica type dielectric capacitors have excellent linearity. Carbon surface mount resistors can generate distortion from self heating and from damage that may occur during soldering. Metal film surface mount resistors are much less susceptible to both problems. Pseudo-Differential Unipolar Inputs For most applications, we recommend the low power LT6202 ADC driver to drive the LTC2369-18. With a low noise density of 1.9nV/√Hz and a low supply current of 3mA, the LT6202 is flexible and may be configured to convert signals of various amplitudes to the 0V to 5V input range of the LTC2369-18. To achieve the full distortion performance of the LTC2369-18, a low distortion single-ended signal source driven through the LT6202 configured as a unity-gain buf- fer as shown in Figure 4 can be used to get the full data sheet THD specification of –120dB. The LT6202 can also be used to buffer and convert large true bipolar signals which swing below ground to the 0V to 5V input range of the LTC2369-18. Figure 5a shows the LT6202 being used to convert a ±10V true bipolar signal for use by the LTC2369-18. In this case, the LT6202 is configured as an inverting amplifier stage, which acts to attenuate and level shift the input signal to the 0V to 5V input range of the LTC2369-18. In the inverting configuration, the single-ended input signal source no longer directly drives a high impedance input. The input impedance is instead set by resistor RIN. RIN must be chosen carefully based on the source impedance of the signal source. Higher values of RIN tend to degrade both the noise and distortion of the LT6202 and LTC2369-18 as a system. Table 1 shows the resulting SNR and THD for several values of RIN, R1, R2, R3 and R4 in this configuration. Figure 5b shows the resulting FFT when using the LT6202 as shown in Figure 5a. Figure 4. Input Signal Chain Another filter network consisting of LPF2 should be used between the buffer and ADC input to both minimize the noise contribution of the buffer and to help minimize distur- bances reflected into the buffer from sampling transients. Long RC time constants at the analog inputs will slow down the settling of the analog inputs. Therefore, LPF2 requires a wider bandwidth than LPF1. A buffer amplifier with a low noise density must be selected to minimize degradation of the SNR. 5.1Ω 10nF 66nF 50Ω LPF2 LPF1 BW = 3.2MHz BW = 48kHz LTC2369-18 IN+ IN– 236918 F04 – + LT6202 VREF 0V |
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