High accuracy analogue signal measurement with MCU(2)

The Intersil ISL28134 Chopper Stabilised Op Amp uses state of the art CMOS design techniques to achieve extremely offset drift and low noise at moderate current levels.

The complex input circuits needed for rail to rail operation often increase both the noise level and the current consumption of an operational amplifier. In the ISL28134, this supply current overhead is minimal and the multiple input devices are exploited to achieve the 8 nV/rt(Hz) input noise level. Low power design throughout the amplifier keeps the total supply current down to 675 microamps (typical). The power can be reduced further to 1.7 mW by operating at 2.5 V supply. The advantage of the chopper stabilised design of the ISL28134 is the near-absence of flicker noise. In low noise, very low frequency applications the total noise is usually dominated by 1/f noise. However in the ISL28134 chopper stabilisation is extensively applied, giving a 1/f noise corner of 0.005Hz (figure 2).

Figure 2: The plot shows the 1/f corner for the ISL28134 at the design parameter of 0.005Hz.

The fact that the plot shows 12 nV/√Hz at 0.001Hz (1000 seconds) shows the stability of the low offset design and performance. The actual data collection for this plot was taken over several hours, so this number at 0.001Hz says the offset is very stable. The ISL28134 is one of the best 'zero drift offset' op amps in the industry today. Moreover, the ISL26102 Delta Sigma Converter also uses chopping techniques to remove 1/f noise and DC errors, preserving the unique 1/f noise performance of the ISL28134. Consequently, the combination of the ISL28134 in differential configuration into the ISL26102 makes for one of the best implementations for precision, low noise design common in bridge sensor and data acquisition applications.

Normally, the data acquisition system designer would be concerned with the chopping frequencies of the two ICs beating together, and would need to synchronise the chopping clock frequencies, or add extra filtering to remove the spurious outputs from the op. amp. None of this complexity is necessary with the ISL28134. A patented filter design reduces the chopping frequency spurious outputs to below the output noise level.

Now let's explore possibilities for data acquisition with low power microcontrollers. In order to achieve low power consumption with microcontrollers, techniques can be applied to periodically sample the sensor or data acquisition inputs. The newer microcontrollers have new provisions to allow for lower power sampling modes for peripheral devices to operate while the microcontroller is in lower power status (table 2).

Modern microcontrollers have additional functions to allow various low power operation modes for external circuits while the microcontroller operates in lowest power modes; for example 0.6 uA of stop or standby current or if single step data transfer is used as low as 45 uA/MHz. Hence this sets the stage to develop periodic sampling techniques of higher power analogue frontend components to reduce power consumption between active and standby modes. For example, the ISL28134 can be left in active mode as low as 1.7 mW (at 2.5 V) while the ISL26102 is at 10?W in standby mode. This technique would allow for a faster and safer response for sensor sampling at lowest power levels and routines can be developed in conjunction of the microcontrollers for independent peripheral operations whilst in lowest power modes.

Table 2: Examples of new modern functions of low power microcontrollers.

Combining the low-power capabilities of the Renesas RL78 MCUs and Energy Micro's Ultra Low Power EFM32 with Intersil's high-precision analogue expertise provides customers with a complete solution to help bring low power hand help applications to market quickly and efficiently. The new AFE-MCU solution is targeted towards industrial, medical and data acquisition applications. Each includes a sensor, signal conditioning circuits, and a microcontroller.

About the author
Alison Evans is with Avnet Memec.

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