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SDM863K Datasheet(PDF) 8 Page - Burr-Brown (TI) |
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SDM863K Datasheet(HTML) 8 Page - Burr-Brown (TI) |
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8 / 27 page  ® SDM862/863/872/873 8 All data acquisition systems using a MUX require consider- ation of the errors that may be introduced by MUX output capacitance. The applications information explains this more fully in the input filtering section. Shown in Figure 3 is an application that demonstrates the flexibility of signal conditioning and gives the opportunity to use a higher bandwidth filter. Diodes shown are low leakage types (1na). The low output impedance of the amplifiers reduces the time taken to charge MUX capaci- tance CM. INSTRUMENT AMPLIFIER The instrument amplifier (INA) presents a very high input impedance to the signal source, eliminating gain errors introduced by voltage divider action between the source output impedance and SDM input impedance. Where the differential models are used, the INA performs the differen- tial to single-ended conversion required to drive the sample/ hold amplifier. Gains may be set by using external jumpers, to values of 1 (no jumper), 10 and 100. For gains other than these presets, the following formula may be used to find an external resistor value to add in series with the G = 10 or G = 100 jumpers. 40k Ω Where Ri = 4444 Ω, G = 10 input. G – 1 404 Ω, G = 100 input. It should be noted that the internal gain set resistors have a ±20% tolerance and ±20ppm/°C drift. to ensure that neither of the differential inputs exceed the maximum input range. Otherwise, signal distortion will result. A return path for the input bias currents must always be provided. This prevents the charging of stray capaci- tances in applications using floating sources, such as trans- formers and thermocouples. Multiplexer inputs are protected from overvoltage, as indicated in the electrical specifica- tions, and should be current limited to 20mA. Where high-speed operation is required and channels require rapid sampling, then it is important to buffer the inputs against the effect of current sharing between the MUX output capacitance and the input filter capacitance. See Figure 2. SYSTEM DESCRIPTION The SDM comprises four circuit elements—an input-pro- tected multiplexer, an instrumentation amplifier, a sample/ hold amplifier, and an analog-to-digital converter. INSTALLATION MULTIPLEXER The SDM family has a choice of input multiplexers (MUX). SDM862 and SDM872: 16 single-ended inputs SDM863 and SDM873: 8 differential inputs On all models, the analog inputs may be expanded using the enable control. See Figure 1. When the enable is at a logic “0,” the internal MUX is disabled, allowing additional multiplexers to be connected in parallel. The limiting factor for the number of additional multiplexers is the cumulative effect of leakage current flowing in the signal source imped- ance, causing offset errors. Differential inputs will generally eliminate the noise associ- ated with common system grounds, but care must be taken FIGURE 1. External Multiplexer Connections for Differen- tial and Single-Ended Operation. FIGURE 2. Filter and MUX Capacitance. – Ri R ext = C f C M C f MUX A3 A2 A1 A0 Out A3 A2 A1 A0 Out A2 A1 A0 – Out +Out 49 50 51 52 48 65 66 53 SDM8X2 66 A2 A1 A0 –Out +Out A0 A1 A2 A3 MUX Extern 49 50 51 48 65 53 SDM8X3 67 A0 A1 A2 A3 INA INA A4 MUX Intern MUX Intern MUX Extern |
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