AD7380/AD7381: What are the pros and cons of a Serial 2 wire mode and serial 1 wire mode?

There are two ways to read the conversion result of the AD7380/AD7381. The conversion result can be read via SDOA and SDOB or via SDOA only . The 2 wire mode allows the conversion result of ADCA via SDOA and the conversion result of ADCB via SDOB. Reading conversion result in 2 wire mode requires 16 SCLK cycles and 14 SCLK cycles for AD7380 and AD7381 respectively. When in 1 wire mode the conversion result of ADC A and ADC B are clocked out in SDOA only and requires 32/28 SCLK cycles for AD7380/AD7381. The one wire mode requires more SCLK cycles that would mean longer time clocking out the conversion result which in effect have a lower throughput rate but the microcontroller will have to interface to only one digital output pin therefore saving pins and complexity. The 2 wire mode requires lesser SCLK cycles which can maximize the throughput rate of the AD7380 but requires two microcontroller I/O pins to read the conversion result.

AD7380/AD7381: What oversampling method is best for an application?

The AD738x has two Oversampling mode, Normal Averaging Oversampling and Rolling Average Oversampling. Normal Averaging is best used in application where a high precision or high SNR or Dynamic range is needed and speed is not a concern, because it can average up to oversampling ratio of 32x but the tradeoff would is that it will have slower throughput rate as the oversampling ratio increases. On the other hand, the rolling average are best fit in application where both the high precision performance and throughput rate are desired. It can have a throughput rate up to 4MSPS and oversampling ratio up to 8x. Additionally in normal averaging the sampling instant of the additional samples is controlled internally by the AD7380. In rolling average the sampling instant is controlled by the user so for applications that are sensitive to the sampling instant rolling average oversampling mode would be more appropriate.

FAQ(AD7380): The AD7380(AD738x family) ADC has an on-chip buffered 2.5V reference. Can this internal reference be use to supply external circuitry?

The buffered 2.5V internal reference of the AD7380 is recommended to use for internal circuitry process. When this on-chip 2.5V reference is use for external circuit, it must be connected to an external buffer amplifier for example the ADA4807. The external buffer allows to provide enough current to supply the exteranl circuity without affecting the AD7380 internal activity.

AD738x low latency feature

Q The AD7380(AD738x family) has a low latency feature, how does this feature benefit the application? A The AD738x conversion result is read on the next cycle following the conversion process. The AD738x family low latency feature allows the latest conversion data to be read immediately as soon as it finishes A to D conversion of 190ns. A minimum of 10ns pulse for CS is required to occur after the conversion time then a high to low transition of CS happens to read the conversion result which requires 16 SCLK cycles for each channel in a 2- wire mode for dual and 4-wire mode quad version of the AD738x family.

AD7380/AD738x 80Mhz SCLK frequency

Q The AD7380/AD738x datasheet uses the 80Mhz SCLK frequency. Can a lower frequency be used in the communication interface? A AD738x has a throughput rate of 4MSPS. Faster SCLK frequency enables to achieve a higher throughput rate. The 80Mhz SCLK frequency enables to achieve of the fast throughput rate of the AD738x of 4MSPS to clock-out the conversion data at the SDO A and SDO B pins. Some applications may use slower throughput, A lower SCLK frequency can be used for the AD738x, like 40Mhz or lower for example, and still able to achieve the ADC excellent performance but then this will have a lower throughput rate to clock-out conversion results in the SDOA and SDOB pins.

AD7380 family Alert function and setting alert threshold

Q The AD7380/AD738x family of generics has an Alert feature.. The limits are set in the Alert threshold register. Can the Alert threshold limit be set per channel? A The alert threshold low and alert threshold high is set in the Alert Threshold Low register and Alert Threshold High register. These thresholds are applicable for all channels of the AD7380 and its family of generics but the alert function is triggered per channel. That means the ALERT register records the channel that triggers the threshold set.

The AD7380(AD738x) a 16-bit resolution ADC throughput rate is at 4MSPS. When resolution boost is enable allowing 18-bit conversion result. With the additional 2-bits, does the AD7380 would still able to maintain the throughput rate at 4MSPS?

The AD7380 is capable to perform with throughput rates over 4MSPS using the correct SCLK at 80 Mhz frequency. When the normal averaging oversampling and resolution boost are enabled, the throughput rate will slow down as the oversampling ratio increases, as this is the tradeoff when using the normal average oversampling. On the otherhand whenever using the rolling average oversampling and resolution boost is enable, even with the additional 2-bit of resolution, the AD7380 is still capable to have a throughput rate of 4MSPS.

The conversion result of the AD738x family generics is clocked-out in the SDOA and SDOB pins. In the typical schematic, these pins have a 100ohm resistor connected going to the digital interface. Can I connect this pin directly without the resistor?

Q The conversion result of the AD738x family generics is clocked-out in the SDOA and SDOB pins. In the typical schematic, these pins have a 100ohm resistor connected going to the digital interface. Can I connect this pin directly without the resistor? A The 100-ohm series resistor in the SDOA and SDOB lines helps manage the effect of transmission. When data transfer has fast rising and falling edges, the performance may be impacted by board layout and design. This can affect the ADC conversion result and can manifest in the SNR parameter. But when running the AD738x at a low speed for example below 40Mhz SCLK, these resistors may not be required.

The AD7380 has an on chip oversampling, Why is there a need for an oversampling?

Question: The AD7380 has an on chip oversampling, Why is there a need for an oversampling? Answer: Oversampling has been a known method of processing conversion result to improve ADC performance for SAR ADC. Putting the oversampling process internally to the AD7380 will have a faster processing time of an oversampled conversion result as compared to performing in the microcontroller which will add processing time and memory, and at the same time it further improves the AD7380 performance.

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