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ADS8168 Datasheet(PDF) 69 Page - Texas Instruments |
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ADS8168 Datasheet(HTML) 69 Page - Texas Instruments |
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69 / 78 page  B REFby2 500 V (V V) 97.5mV 10k 500 : § · u ¨ ¸ : : © ¹ ADC Sequencer AIN0 AIN1 AIN7 + AIN0 MUXOUT-P ADC-INP OPA320 ADS8168 Solution ± Single Wide Bandwidth Amplifier 43.2k 5V 3.6pF 49.9 10k 0µA to 90µA 3.6pF 4.096V ÷2 VREF 500 VB = 0.1V REFby2 VB = 0.1V ADS8168 AIN1 AIN7 SFH213 0µA to 90µA 69 ADS8168 www.ti.com SBAS817 – NOVEMBER 2017 Product Folder Links: ADS8168 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated 8.2.3 8-Channel Photodiode Detector With Smallest Size and Lowest Number of Components The circuit in Figure 79 shows an 8-channel photodiode detector using the ADS8168. In this example, one common amplifier is used for eight photodiodes. A detailed description of the transimpedance amplifier is given in TI Precision Designs. Figure 79. Small Size, 8-channel Photo Detector 8.2.3.1 Design Requirements The objective of this design is to achieve: • Smallest solution size. • Transimpedance output of 0.1-V to 4-V for a 0-µA to 90-µA input with a bandwidth of 1-MHz. • The voltage divider is designed to provide a minimum amplifier output of 0.1-V when the photodiode current is zero (dark current). This prevents the amplifier from saturating to the negative rail. 8.2.3.2 Detailed Design Procedure In Figure 79, the photodiodes are connected to the multiplexer input in photovoltaic mode. Depending on the application requirements, either photovoltaic mode or photoconductive mode can be used. The multiplexer in the ADS8168 is used as a current multiplexer in this example. One common amplifier for all photodiodes reduces cost, complexity, PCB area, and power consumption. It also simplifies system calibration as the gain and offset error is the same for all channels. Finally, the low leakage current of the multiplexer is ideal for photodiode applications OPA320 is used as a transimpedance amplifier that can also drive the ADC inputs. In order to set the output voltage of OPA320 to 0.1-V in dark conditions, an equivalent bias voltage (VB) is applied at the non-inverting terminal. This bias voltage is derived using a resistive voltage divider on REFby2 output (2.048V), as shown in Equation 14. (14) The feedback resistor for the transimpedance amplifier can be selected by designing for 4-V output for a 90-µA input, as shown in Equation 15. |
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