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AD5934 Datasheet(PDF) 21 Page - Analog Devices |
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AD5934 Datasheet(HTML) 21 Page - Analog Devices |
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21 / 31 page 
Data Sheet AD5934 Rev. E | Page 21 of 31 Control Register Decode Initialize with Start Frequency This command enables the DDS to output the programmed start frequency for an indefinite time. Initially, it is used to excite the unknown impedance. When the output unknown impedance has settled after a time determined by the user, the user must initiate a start frequency sweep command to begin the frequency sweep. Start Frequency Sweep In this mode, the ADC starts measuring after the programmed number of settling time cycles has elapsed. The user has the ability to program an integer number of output frequency cycles (settling time cycles) to Register Address 0x8A and Register Address 0x8B before the commencement of the measurement at each frequency point (see Figure 24). Increment Frequency The increment frequency command is used to step to the next frequency point in the sweep. This usually happens after data from the previous step is transferred and verified by the DSP. When the AD5934 receives this command, it waits for the programmed number of settling time cycles before beginning the ADC conversion process. Repeat Frequency There is the facility to repeat the current frequency point measurement by issuing a repeat frequency command to the control register. This command allows users to average successive readings. Power-Down Mode The default state at power-up of the AD5934 is power-down mode. The control register contains the code 1010,0000,0000,0000 (0xA000). In this mode, both the output and input pins, VOUT and VIN, are connected internally to GND. Standby Mode This mode powers up the part for general operation. In standby mode, the VIN and VOUT pins are internally connected to GND. Reset A reset command allows the user to interrupt a sweep. The start frequency, number of increments, and frequency increment register contents are not overwritten. An initialize with start frequency command is required to restart the frequency sweep command sequence. Output Voltage Range The output voltage range allows the user to program the excitation voltage range at VOUT. PGA Gain The PGA gain allows the user to amplify the response signal into the ADC by a multiplication factor of ×5 or ×1. START FREQUENCY REGISTER (REGISTER ADDRESS 0x82, REGISTER ADDRESS 0x83, REGISTER ADDRESS 0x84) The start frequency register contains the 24-bit digital representation of the frequency from where the subsequent frequency sweep is initiated. For example, if the user requires the sweep to start from a frequency of 30 kHz using a 16.0 MHz clock, the user must program the value 0x3D to Register Address 0x82, the value 0x70 to Register Address 0x83, and the value 0xA3 to Register Address 0x84. Doing this ensures the output frequency starts at 30 kHz. The start frequency code is 0x3D70A3 2 16 MHz 16 kHz 30 27 ≡ × = Code Frequency Start The default value of the start frequency register upon reset is as follows: D23 to D0 are not reset at power-up. After the reset command, the contents of this register are not reset. FREQUENCY INCREMENT REGISTER (REGISTER ADDRESS 0x85, REGISTER ADDRESS 0x86, REGISTER ADDRESS 0x87) The frequency increment register contains a 24-bit representation of the frequency increment between consecutive frequency points along the sweep. For example, if the user requires an increment step of 30 Hz using a 16.0 MHz clock, the user must program the value 0x00 to Register Address 0x85, the value 0x0F to Register Address 0x86, and the value 0xBA to Register Address 0x87. The formula for calculating the frequency increment is given by 0x00053E 2 16 MHz 16 Hz 10 27 ≡ × = Code Increment Frequency The user programs the value 0x00 to Register Address 0x85, the value 0x05 to Register Address 0x86, and the value 0x3E to Register Address 0x87. The default value of the frequency increment register upon reset is as follows: D23 to D0 are not reset at power-up. After the reset command, the contents of this register are not reset. |
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