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AD8233CB-EBZ Datasheet(PDF) 6 Page - Analog Devices |
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AD8233CB-EBZ Datasheet(HTML) 6 Page - Analog Devices |
6 / 12 page UG-1016 AD8233CB-EBZ User Guide Rev. 0 | Page 6 of 12 For three-electrode dc mode, each input must have a pull-up resistor connected to the positive supply. During normal operation, the potential of the subject must be inside the common-mode range of the instrumentation amplifier, which is only possible if a third electrode is connected to the output of the RLD amplifier. To create this connection, place jumpers at P4, P6, and P7. RLD must also be enabled by putting the S4 switch in the RLD_EN position. RLD 10MΩ 10MΩ TO DRIVEN ELECTRODE IA B5 C5 D5 +VS Figure 7. Circuit Configuration for Three-Electrode DC Leads Off Detection The AD8233 indicates when any electrode is disconnected by setting the LOD pin high. AC Leads On/Off Detection The ac leads on/off detection mode is useful when using two- electrodes. To use the ac leads on/off mode, switch the S2 switch to the AC position. In this case, a conduction path must exist between the two-electrodes, which is usually formed by two resistors, shown in Figure 8. These resistors also provide a path for bias return on each input. Connect each resistor (place jumpers at P6 and P7) to the REFOUT pin (place a jumper at P3) or RLD pin (place a jumper at P5) to maintain the inputs within the common-mode range of the instrumentation amplifier. If the resistors are tied to the RLD pin, the S4 switch must be in the RLD_EN position. REFOUT 10MΩ 10MΩ IA B5 C5 A2 D3 +VS Figure 8. Circuit Configuration for Two-Electrode AC Leads Off Detection The AD8233 detects when an electrode is disconnected by forcing a small 100 kHz current into the input terminals. This current flows through the external resistors from the +IN pin to the −IN pin and develops a differential voltage across the inputs, which is then synchronously detected and compared to an internal threshold. The recommended value for these external resistors is 10 MΩ. Low resistance values make the differential drop too low to be detected and lower the input impedance of the amplifier. When the electrodes are attached to the subject, the impedance of this path must be less than 3 MΩ to maintain the drop below the comparator threshold. While the REFOUT pin is at a constant voltage value, using the RLD output as the input bias may be more effective in rejecting common-mode interference at the expense of additional power. In three-electrode ac leads off detection mode, shown in Figure 9, pull-up resistors are not required, which imporves the input impedance of the circuit. This mode is beneficial for dry electrode applications. The ac mode currents do contribute 1/f noise to the system; therefore, depending on the application, it can be advantageous to utilize ac leads off as a spot check and switches to dc mode for improved ECG acquisition. To create this connection, remove the Input Bias Selection jumpers and the Electrode Terminals jumpers. RLD IA B5 C5 A2 D5 +VS TO DRIVEN ELECTRODE Figure 9. Circuit Configuration for Three-Electrode AC Leads Off Detection The ac leads off detection mode continues to function in shutdown mode. To keep power under 1 µA, the clock is disabled and the ac currents become dc currents. The current source on the +IN pin is 250 nA, while the current sink on the –IN pin is –300 nA. The stronger pull-down current on –IN acts as a wake-up function, pulling the LOD pin low when the electrodes are reconnected. |
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