7.3.1 Serial Interface

An operational timing diagram and a serial interface timing diagram for the ADC128S022 are shown in the Specifications section. CS, chip select, initiates conversions and frames the serial data transfers. SCLK (serial clock) controls both the conversion process and the timing of serial data. DOUT is the serial data output pin, where a conversion result is sent as a serial data stream, MSB first. Data to be written to the ADC128S022's Control Register is placed on DIN, the serial data input pin. New data is written to DIN with each conversion.

A serial frame is initiated on the falling edge of CS and ends on the rising edge of CS. Each frame must contain an integer multiple of 16 rising SCLK edges. The ADC's DOUT pin is in a high impedance state when CS is high and is active when CS is low. Thus, CS acts as an output enable. Similarly, SCLK is internally gated off when CS is brought high.

During the first 3 cycles of SCLK, the ADC is in the track mode, acquiring the input voltage. For the next 13 SCLK cycles the conversion is accomplished and the data is clocked out. SCLK falling edges 1 through 4 clock out leading zeros while falling edges 5 through 16 clock out the conversion result, MSB first. If there is more than one conversion in a frame (continuous conversion mode), the ADC will re-enter the track mode on the falling edge of SCLK after the N*16th rising edge of SCLK and re-enter the hold/convert mode on the N*16+4th falling edge of SCLK. "N" is an integer value.

The ADC128S022 enters track mode under three different conditions. In Figure 1, CS goes low with SCLK high and the ADC enters track mode on the first falling edge of SCLK. In the second condition, CS goes low with SCLK low. Under this condition, the ADC automatically enters track mode and the falling edge of CS is seen as the first falling edge of SCLK. In the third condition, CS and SCLK go low simultaneously and the ADC enters track mode. While there is no timing restriction with respect to the rising edges of CS and SCLK, see Figure 3 for set-up and hold time requirements for the falling edge of CS with respect to the rising edge of SCLK.

While a conversion is in progress, the address of the next input for conversion is clocked into a control register through the DIN pin on the first 8 rising edges of SCLK after the fall of CS. See Table 1, Table 2, and Table 3.

There is no need to incorporate a power-up delay or dummy conversion as the ADC128S022 is able to acquire the input signal to full resolution in the first conversion immediately following power up. The first conversion result after power up will be that of IN0.

7.3.2 ADC128S022 Transfer Function

The output format of the ADC128S022 is straight binary. Code transitions occur midway between successive integer LSB values. The LSB width for the ADC128S022 is V_A / 4096. The ideal transfer characteristic is shown in Figure 36. The transition from an output code of 0000 0000 0000 to a code of 0000 0000 0001 is at 1/2 LSB, or a voltage of V_A / 8192. Other code transitions occur at steps of one LSB.

Figure 36. Ideal Transfer Characteristic

7.3.3 Analog Inputs

An equivalent circuit for one of the input channels of the ADC128S022 is shown in Figure 37. Diodes D1 and D2 provide ESD protection for the analog inputs. The operating range for the analog inputs is 0 V to V_A. Going beyond this range will cause the ESD diodes to conduct and result in erratic operation.

The capacitor C1 in Figure 37 has a typical value of 3 pF and is mainly the package pin capacitance. Resistor R1 is the ON-resistance of the multiplexer and track / hold switch and is typically 500 Ω. Capacitor C2 is the ADC128S022 sampling capacitor, and is typically 30 pF. The ADC128S022 will deliver best performance when driven by a low-impedance source (less than 100 Ω). This is especially important when using the ADC128S022 to sample dynamic signals. Also important when sampling dynamic signals is a bandpass or lowpass filter which reduces harmonics and noise in the input. These filters are often referred to as anti-aliasing filters.

Figure 37. Equivalent Input Circuit

7.3.4 Digital Inputs and Outputs

The digital inputs of the ADC128S022 (SCLK, CS, and DIN) have an operating range of –0.3 V to V_A. They are not prone to latch-up and may be asserted before the digital supply (V_D) without any risk. The digital output (DOUT) operating range is controlled by V_D. The output high voltage is V_D – 0.5V (minimum) while the output low voltage is 0.4 V (maximum).

Table 1. Control Register Bits

Table 2. Control Register Bit Descriptions

Table 3. Input Channel Selection