10.4.1.3 4-Wire CS Mode Without a Busy Indicator

This interface option is useful when one or more ADCs are connected to an SPI-compatible digital host. Figure 52 shows the connection diagram for single ADC; see Figure 54 for the connection diagram for two ADCs.

In this interface option, DIN is controlled by the digital host and functions as CS. As shown in Figure 53, with DIN high, a CONVST rising edge selects CS mode, forces DOUT to 3-state, samples the input signal, and causes the device to enter a conversion phase. In this interface option, CONVST must be held at a high level from the start of the conversion until all data bits are read. Conversion is done with the internal clock and continues regardless of the state of DIN. As a result, DIN (functioning as CS) can be pulled low to select other devices on the board. However, DIN must be pulled high before the minimum conversion time (t_conv-min) elapses and remains high until the maximum possible conversion time (t_conv-max) elapses. A high level on DIN at the end of the conversion ensures the device does not generate a busy indicator.

When conversion is complete, the device enters acquisition phase and powers down. DIN (functioning as CS) can be brought low after the maximum conversion time (t_conv-max) elapses. On the DIN falling edge, DOUT comes out of 3-state and the device outputs the MSB of the data. The lower data bits are output on subsequent SCLK falling edges. Fast sampling rates require high frequency SCLK and data must be read at SCLK falling edges. For slow sampling rates and SCLK frequency ≤ 36 MHz, data can be read at either SCLK falling or rising edges. Note that with any SCLK frequency, reading data at SCLK falling edge requires the digital host to clock in the data during the t_{h_CK_DO-min} time frame. DOUT goes to 3-state after the 16th SCLK falling edge or when DIN goes high, whichever occurs first.

As shown in Figure 54, multiple devices can be hooked together on the same data bus. In this case, as shown in Figure 55, the DIN of the second device (functioning as CS for the second device) can go low after the first device data are read and the DOUT of the first device is in 3-state.

Care must be taken so that CONVST and DIN are not both low together at any time during the cycle.