±10V analog input and ±15V analog output I/O module for Raspberry Pi

±10V analog input and ±15V analog output I/O module for Raspberry Pi

While the world continues to become more digital, computing power and digital functions are becoming more critical, the need to measure the environment and interact with real devices remains an analog function. In order to operate at the boundary of the digital and analog domains, processors must include mixed-signal input/output and accommodate more software programmability to support many industrial, instrumentation, and automation applications.

The circuit shown in Figure 1 is a flexible, multichannel, mixed-signal analog input/output (I/O) module. The 16 single-ended analog outputs are software configurable to support ranges of 0 V to 5 V, ±5 V, 0 V to 10 V, and ±15 V. The eight fully differential analog input channels have hardware selectable input ranges of 0 V to 2.5 V, ±13.75 V, and 0 V to 27.5 V.

Figure 1. ADI CN0554 simplified functional block diagram

The circuit mounts directly on top of the Raspberry Pi, providing an analog I/O interface to this popular single-board computer. Software control is accessible through the Linux Industrial Input/Output (IIO) framework, which provides a variety of debugging and development utilities, as well as a cross-platform application programming interface (API) that supports language bindings such as C, C#, MATLAB, Python, and more.

The software can be run locally on the Raspberry Pi or remotely controlled via a wired or wireless network connection. The module's 5V power supply is provided through the Raspberry Pi interface connector and no additional power supply is required. All of these features make the system suitable for low-power, local and remote, precision analog I/O applications.

Evaluation and Design Support

►Circuit Evaluation Board

► ADI CN0554 circuit evaluation board (EVAL-CN0554-RPIZ)

► Design and integration documentation

►Schematics , layout files, bill of materials, software

Circuit Description

The ADI CN0554 provides a complete analog I/O system for precision applications. The circuit can be broken down into two main components: analog-to-digital conversion (ADC) and digital-to-analog conversion (DAC).

Analog input

The CN0554 can accommodate analog input voltage ranges up to 11x input attenuation through the on-board LT5400 external matching resistor network. The device increases the analog input voltage range through jumper selection.

Table 1 shows the complete jumper configuration and corresponding analog input voltage range for the CN0554.

Table 1. Analog Input Ranges

Analog -to-digital conversion

The CN0554 contains the AD7124-8, a 24-bit Σ-Δ ADC with configurable analog inputs. The eight fully differential input channels or 16 single-ended input channels are software configurable with programmable gain, filter settings, and output data rates.

The external reference voltage can be selected via a jumper and can be either the internal bandgap reference of the AD7124-8 or the 2.5V output of the ADR4525, a high precision, low power, and low noise reference. Since the reference voltage drift directly affects the accuracy of the ADC, the CN0554 uses an external reference because it has better temperature drift performance than the internal reference.

The Σ-Δ architecture of the AD7124-8 provides high resolution and noise rejection in small signal sensor measurements, even in high noise environments such as industrial. The output data rate is programmable from 1.17SPS to 19.2kSPS, with corresponding measurement resolutions of 24nV rms to 72μV rms; several filter modes are available. This allows the resolution, data rate, and noise rejection of the CN0554 to be optimized for a wide range of applications.

Digital to analog conversion

The CN0554 contains 16 single-ended 16-bit analog outputs using the LTC2688 voltage output DAC. Each channel has an internal rail-to-rail output buffer and can source or sink up to 20mA.

The LT8582 provides the ±18V rails for the LTC2688, allowing the DAC to fully utilize its analog output range of up to ±15V. The output range of each channel can be independently programmed to one of the five ranges listed in Table 2. The reference voltage is software programmable to use the internal 4.096V or the ADR4525 2.5V reference for the ADC. Each channel also supports 5% overrange.

Table 2. Voltage Output Range Adjustment

Switching and jittering functions

The CN0554 supports both toggle and dither functions. The toggle function can quickly switch the DAC output between two different DAC codes without any SPI transactions, thus eliminating communication transactions. Examples include injecting a small DC bias or switching independently between on and off states.

Dithering reduces quantization errors in precision applications and does so by spreading nonlinearity over multiple output codes. This feature is helpful in many applications where it is necessary to superimpose an AC signal near the average DC value of the signal. For example, in optical applications, the secondary characteristics of an optical path can be measured by its response to a small AC signal. In addition, dithering reduces stiction in mechanical systems such as spool valves, speeding up the response when the spool position changes.

Switch operation

As shown in Figure 2, each channel supports switching operation, which can switch the output voltage between two values ​​set by software. The switching is controlled by a switching signal, which can be obtained from three different external digital inputs (TGP0, TGP1, and TGP2) of the Raspberry Pi or internal software-controlled registers. Two of the digital signals, TGP0 and TGP1, are connected to the Raspberry Pi digital outputs and support pulse width modulation (PWM).

Figure 2. Block diagram of switching and dithering operation

An example of the switching operation performed by the CN0554 is shown in Figure 3. Depending on the switching pin, the output voltage swings between zero scale and full-scale value, with a peak-to-peak voltage of 33.0V measured at 1kHz.

Figure 3. Zero-scale to full-scale output voltage switching

Jitter operation

In the CN0554, each channel also supports dithering, which adds a sinusoidal dithering signal to the analog output. The sinusoid is generated using a lookup table whose values ​​come from Equation 1.

(1)

in:

n = 0, 1, 2, … N — 1。

N is the signal period.

φ0 is the signal phase angle, the initial signal phase.

CN0554 can configure the amplitude, period, and phase angle of the dither signal.

The amplitude of the dither signal is set by software and can be between 0% and 25% of the set maximum output voltage.

In order to set the dither frequency, a dither clock input is required and can be selected from the three external digital inputs of the Raspberry Pi, TGP0, TGP1, and TGP2. Two of the external inputs, TGP0 and TGP1, are connected to the Raspberry Pi digital outputs and have PWM characteristics, which can easily configure the clock frequency.

The frequency of the dither signal is set by the dither clock input divided by a software configurable divider of 4, 8, 16, 32, and 64, resulting in the frequency of the dither signal being calculated using Equation 2:

(2)

in:

fsignal is the frequency of the generated jitter signal.

fPWM is the PWM clock frequency.

N is the divider.

The dither phase angle can be configured to four different values: 0, 90, 180, and 270. All of these parameters help to precisely control the dither DAC channel output.

Figure 4 shows an example of the dithering operation performed by the CN0554 at the mid-scale output voltage of the maximum signal period, with a peak-to-peak voltage of 15.04V at a 1kHz dithered clock.

Figure 4. Mid-scale output voltage at maximum signal period

Figure 5 shows the dithering operation performed at the mid-scale output voltage of the minimum signal period, with a peak-to-peak voltage of 17.6V at a 1kHz dithering clock.

Figure 5. Mid-scale output voltage at minimum signal period.

System performance

Analog input noise performance

Figure 6 shows the noise characteristics at midscale input (5V), and Figure 7 shows the noise characteristics at full-scale input (10V).

Figure 6. Midscale analog input noise histogram

Figure 7. Full-Scale Analog Input Noise Histogram

Analog output noise performance

The switching regulator output of the LT8582 is bypassed and filtered to reduce noise. Figure 8 shows the AC-coupled signal noise at zero-scale output, which has very low peak-to-peak noise at 14.4mV.

Figure 8. Zero-scale AC-coupled noise signal from ADC and DAC channel loopback

Figure 9 shows the 13.4mV peak-to-peak noise generated at midscale output.

Figure 9. Mid-level AC-coupled noise signal from ADC and DAC channel loopback

In Figure 10, the board produces a maximum peak-to-peak noise of 17.6mV at full-scale output.

Figure 10. Full-scale AC-coupled noise signal from ADC and DAC channel loopback

Analog output linearity

Integral nonlinearity (INL) is the maximum deviation (in LSBs) from a straight line through the endpoints of the DAC transfer function. Additionally, differential nonlinearity (DNL) is the difference between the measured change and the ideal 1LSB change between any two adjacent codes. A maximum ±1LSB differential nonlinearity rating ensures monotonicity.

Figure 11 shows the DNL (in LSB) of the output voltage versus the 16-bit setting value for a single LTC2688 output.

Figure 11. Differential nonlinearity of output voltage

Figure 12 shows the INL (in LSB) of the output voltage versus the 16-bit setting value for a single LTC2688 output.