ΔΣ ADC circuit reduces design complexity and time

Delta-Sigma (Δ-Σ) analog-to-digital converters (ADCs) with Easy Drive technology are not only feature-rich, but also easy to use. The Easy Drive feature simplifies or eliminates active amplification or filtering at the input , and the software interface is much simpler than other types of ADCs. It eliminates the complexity of traditional ADC application design (such as the need for external components and software), which can significantly save design time.

Table 1 shows the features of the 18 available Easy Drive devices, including single-, 4-, or 16-channel versions with I2C or SPI interfaces. The 24-bit devices are suitable for very high-performance applications, while the 16-bit devices are more versatile. A programmable gain amplifier (PGA) is also available on the 16-bit devices to meet intermediate requirements or for use in applications that need to accommodate multiple input ranges.

Simplifying high impedance sensor measurement circuits

Delta-Sigma ADCs are highly accurate and noise-immune, making them ideal for directly measuring many types of sensors. However, their input sampling currents require either high source impedance or low bandwidth, micropower signal conditioning circuits. Easy Drive technology solves this problem by balancing the input currents, which can simplify or eliminate the need for signal conditioning circuits.

Table 1: Complete Easy Drive delta-sigma ADC family.

A common application for delta-sigma ADCs is the measurement of thermistors . Figure 1 shows two examples of digital thermistor measurements that benefit from Easy Drive technology. The first circuit (applied to input channels CH0 and CH1) uses two equal-value reference resistors to set the input common-mode voltage equal to the reference common-mode voltage and balance the differential input source resistance. If the reference resistors R1 and R4 are exactly equal, the input current is zero and no error is introduced. If these resistors have a 1% tolerance, the error in the measured resistance caused by the large common-mode voltage shift is a maximum of 1.6Ω, which is much less than the 1% error of the reference resistors themselves. This is an ideal solution in micropower applications because no amplifier is required.

Figure 1: The Easy Drive ADC simplifies the measurement circuit for high impedance sensors.

Easy Drive technology allows low power, low bandwidth amplifiers to drive the input of the LTC2492. As shown in Figure 1, CH2 is driven by the LT1494 . For an amplifier with a 1.5μA supply current, the LT1494 has excellent DC parameters, providing a maximum offset voltage of 150μV and an open-loop gain of 100,000. However, the bandwidth of the LT1494 is only 2kHz, which is not suitable for driving traditional delta-sigma ADCs. This problem can be solved by adding a filter with R=1kΩ and C=0.1μF. This filter forms a charge reservoir that provides instantaneous current to the LTC2492, while the 1kΩ resistor isolates the capacitive load from the LT1494. At this time, the input sampling current of the traditional delta-sigma ADC generates a DC error because the external RC network is not completely stable. Linear Technology's Easy Drive technology cancels the differential input current. By balancing the negative input (CH3) with an RC network of R = 1kΩ, C = 0.1μF, the error due to the common-mode input current is eliminated.

Figure 2: In this circuit, an external buffer provides a high-impedance input and the amplifier's offset is automatically cancelled.

Complete Easy Drive Delta-Sigma ADC Family

Easy Drive ADCs have been widely used in many applications. The 24-bit, 16-channel LTC2498 with integrated temperature sensor is ideal for high-performance data acquisition systems. It can directly digitize thermocouples without any signal conditioning circuitry and provide cold junction compensation. It can also directly measure low-level strain gauge outputs and measure industrial sensor voltages by adding a simple resistive voltage divider (no active circuitry required).

The 16-bit, 16-channel devices are ideal for measuring voltage and current on large circuit boards with multiple high-current power supplies. If the COM pin is tied to a common ground point, up to 16 ground-referenced measurements can be made. The differential inputs (up to 8 differential input channels) allow high-side sensing of current shunts as long as the shunt common-mode voltage is less than or equal to the ADC supply voltage. Differential measurements also allow remote sampling of the voltage, eliminating errors caused by large ground currents.

Figure 3: SPI interface, configuration, and data output timing diagram.

Another important advantage of using a delta-sigma ADC to measure supply voltages is its strong immunity to noise and switching transients. The ADC's internal SINC4 filter (along with a simple single-pole filter at the ADC input) is sufficient to attenuate switching supply noise below the ADC noise floor, allowing an accurate DC measurement of the supply voltage or current.

The single-channel LTC2482 is ideal for cost-sensitive applications such as portable medical equipment and consumer products. Despite its relatively low cost, it is essentially a high-performance 16-bit ADC with the same 600nV input noise floor as a 24-bit device.

Figure 4: I2C conversion sequence.

Automatic offset calibration of external buffers/amplifiers

In addition to the Easy Drive input current cancellation feature, the 16-bit Easy Drive ADC allows an external amplifier to be placed between the multiplexer output and the ADC input (Figure 2). This is useful in applications where balanced source impedances are not possible or where the source impedances are very high. All 17 ...

Figure 5: I2C configuration and data output timing diagram.

The LTC6078 is an ideal amplifier for this function. It operates from a voltage as low as 2.7V and has a very low voltage noise level of only 18nV/√Hz. The Easy Drive input of the LTC2498 allows the RC network to be placed directly at the output of the LTC6078. The capacitor reduces the current spikes at the ADC input, while the resistor isolates the capacitive load from the op amp output to maintain stable operation.

Software Interface

The simplicity of analog interface requirements for Linear Technology's Easy Drive ADCs is matched by the simplicity of their serial interface requirements. The no-latency architecture eliminates the annoyance of having to delete readings after channel switching on multi-channel devices. The start of conversions is controlled directly by the serial interface , allowing external signal conditioning circuits or sensor excitation circuits to be connected at the correct point in time. Implicit offset and gain calibration inherent in each conversion cycle eliminates the need for complex internal register sets or calibration cycles. Communication for both SPI and I2C interface devices is a simple read/write operation. In this operation, the data from one conversion cycle is read out while the configuration of the next channel is programmed and written to the ADC.

Figure 6: This circuit quickly selects which SDI word corresponds to each input channel.

Figure 3 shows the data input/output operation of the LTC2498. This is the ADC with the most channels and features with an SPI interface, and other SPI devices have similar interfaces. Figure 4 shows the data input/output operation of the LTC2499. It is also the I2C device with the most features, and other I2C devices have similar interfaces. Figure 5 shows the details of writing the channels and configurations to the input registers.

尽管Easy Drive串行接口易于编程(只需读出用于样本N的数据并设置用于样本N+1的通道)，但是，当通过调试程序来检查某个微控制器的寄存器时，想弄清楚刚刚读出的是什么信息仍颇为棘手。下面介绍一种能显著降低代码设计难题的硬件方法。在图6电路中，每个单端输入上都施加了一个已知电压。当采用图中给出的器件参数时，CH0具有101mV电压，CH1具有202mV电压，依此类推，直到CH15(它具有1.616V电压)。图7是用于差分输入的等效电路。采用这种电路方案，可迅速选出哪个SDI字与每个输入通道相对应。

C-code drivers for basic LTC2448 and LTC2449 communications.

C-code drivers for basic LTC2448 and LTC2449 communications.