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AN-CM-308 Analog Front End for a Pressure Sensor

Contents

Terms and Definitions

AFE Analog front end
IC Integrated circuit
OpAmp Operational amplifier
RH Digital rheostat

References

For related documents and software, please visit:

https://www.dialog-semiconductor.com/advanced-analog-products

Download our free GreenPAK Designer software [1] to open the .gp files [2] and view the proposed circuit design. Use the GreenPAK development tools [3] to freeze the design into your own customized IC in a matter of minutes. Dialog Semiconductor provides a complete library of application notes [4] featuring design examples as well as explanations of features and blocks within the Dialog IC.

  1. GreenPAK Designer Software, Software Download and User Guide, Dialog Semiconductor
  2. AN-CM-308 Analog Front End for Pressure Sensor.gp, GreenPAK Design File, Dialog Semiconductor
  3. GreenPAK Development Tools, GreenPAK Development Tools Webpage, Dialog Semiconductor
  4. GreenPAK Application Notes, GreenPAK Application Notes Webpage, Dialog Semiconductor
  5. SLG47004, Datasheet, Dialog Semiconductor

Author: Vladyslav Kozlov

Introduction

In the following application note the SLG47004 is used as the analog front-end (AFE) for a Wheatstone bridge pressure sensor. Two configurable OpAmps and one internal OpAmp, all within the SLG47004 are used to create an instrumentation amplifier. Digital rheostats RH0 and RH1 of the SLG47004 are used to tune the gain of the AFE and to compensate offset voltages of sensor and OpAmps. Also, the trim procedure helps to minimize an error caused by mismatch between external resistors. The Auto-Trim function of the SLG47004 simplifies the process of gain tuning and offset compensation and allows saving hardware resources, as well as minimize the cost of the AFE.

The SLG47004 allows two different ways of interfacing ADCs with and without internal reference sources:

  • In the case when an external ADC has a separate analog reference pin, the common way is to supply all analog blocks (sensor, ADC, DAC for compensating offset) from one voltage source. The measurements are ratiometric. Variations in supply voltage don’t affect accuracy.
  • In the case when an ADC has an internal reference source only, the supply voltage for the sensor and DAC must be stable and constant. That’s why, for this case, an internal buffered Vref of the SLG47004 must be used.

AFE Without Internal Voltage Reference Source

Hardware Setup of AFE Without Internal Voltage Reference

Figure 1 shows a schematic of the analog front end for MCU with ADC, which has an external analog reference option. Sensor, ADC reference, DAC (Rdiv1, RH0, Rdiv2 divider for offset compensation), and Chopper ACMP reference are powered from one voltage source: Van. Characteristics of the components can be found in Table 1. A pressure sensor from Honeywell (NSCSDRN060MD) is used in this example.

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Figure 1: Analog Front-End for a Wheatstone Bridge Sensor

In Figure 1 Van is the supply voltage for analog components. Van is filtered VDD voltage. The output of the sensor with no pressure is equal to (Van/2 ± Vos_bridge), where Vos_bridge is the bridge offset voltage. Since the force to the sensor can be applied in both directions, the output of the sensor can be higher or lower than zero point (Van/2 ± Vos_bridge). So the AFE must amplify the output signal differentially between the sensor zero point and its actual output.

The optional Cf capacitance is needed to cancel the switching noise of digital rheostats. The value of Cf can be changed.

The output voltage of AFE is:

where

Rf – are user-defined resistors, Rf = 200 KΩ and 0.5% tolerance in the current project;

Rg – is user-defined gain resistor;

Vref – is the reference voltage for the instrumentation amplifier.

Precision Characteristics of Components

Precision characteristics of the components are shown in Table 1.

Table 1: Precisive Characteristics of Components
Parameter
Description
Value
Unit
Sensor Characteristics
ΔP
Pressure Range
±6.0
kPa
KVout
Full Scale Span Coefficient
±2.46 (min), ±2.60 (typ), ±2.8 (max)
mV/V
Kos_out
Null Offset Coefficient
±0.075
mV/V
Sensor Characteristics at 3.3 V DC
ΔVout
Output Voltage Span
±8.1 (min), ±8.58 (typ), ±9.2 (max)
mV
Vos_bridge
Null Offset
±0.248 (max)
mV
dVos_bridge/dT
Offset Temperature Drift (T = 0 to 50 °C)
±0.6 (max)
%FSS
OpAmps Characteristics
VosOpAmp
Input Offset Voltage
1.0 (max)
mV
dVosOpAmp/dT
Offset Drift with Temperature
5 (max)
µV/°C
ΔRint
Mismatch Between Internal R1, R2, R3, R4 Resistors
0.05
%
Digital Rheostats Characteristics
RH1, RH2
Digital Rheostats Resistance
80 (min), 100 (typ), 120 (max)
kOhm
Ntaps
Number of Taps
1024
fChACMP
Chopper Comparator Switching Frequency
10
kHz
VCh_offset
Chopper Comparator Offset when Set is Active
300 (max)
µV
DNL
Differential Non-Linearity (max)
1
LSB
αR(T)
Nominal Resistance Temp Coefficient
100
ppm/°C
HD Buffer Characteristics
Vos_HD_Buf
HD Buffer Offset
±3 (max), T = 25 °C
mV
ΔVOUT(I)
HD Buffer Load Regulation at ILoad = 2mA
2
mV
External Resistors Characteristics
ΔRext
Resistors Tolerance
0.5 and 1
%
αRext(T)
Resistance Temp Coefficient
50
ppm/°C

Internal GreenPAK Design and Macrocells Configurations

Internal Design of the Project

Figure 2 shows the internal design of the project in GreenPAK Designer Software.

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Figure 2: Internal Design of the Project

OpAmps Configurations

OpAmps configurations are shown in Figure 3.

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Figure 3: OpAmps Configurations

Chopper ACMP Configuration

Channel0 of Chopper ACMP is used for offset correction. Channel1 of Chopper ACMP is used for tuning gain of AFE. Chopper ACMP configuration is shown in Figure 4.

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Figure 4: Chopper ACMP Configuration

Digital Rheostats Configurations

Digital Rheostats configurations are shown in Figure 5.

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Figure 5: Digital Rheostats Configurations

LUT Configuration

LUT configuration is shown in Figure 6.

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Figure 6: LUT Configuration

Temperature Sensor Configuration

The temperature sensor configuration is shown in Figure 7.

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Figure 7: Temperature Sensor Configuration

Oscillator0 and I2C Macrocells Configurations

Oscillator0 and I2C Macrocells use default configurations.

GPIOs Configurations

GPIOs configurations are shown in Figure 8.

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Figure 8: GPIOs Configurations

Gain Resistor Calculation

To calculate the value of the gain resistor Rg the minimum and maximum gain of the instrumentation amplifier must be assessed. Considering the possible output span of the sensor Van·KVout (from 8.12 mV to 9.24 mV for Van = 3.3 V), the gain of AFE can be found from the equation:

where Gain_ref_ChopACMP – is the reference voltage of ChopperACMP for gain tuning (see Channel1 In- reference source of Chopper ACMP, Section 4.3.3). Gain_ref_ChopACMP = VDDA*(3/64) or 0.155 V for Van = 3.3 V.

For the schematic shown in Figure 1 VDDA = Van. So, the equation (2) can be rewritten as

or

From equation (3) it’s seen that variations in Van voltage don’t affect the gain of system:

Now it’s possible to build the graph for the function Gain = f(n), where n – is the code of the rheostat from 1 to 1024:

Note that the chip to chip variation of RH maximum resistance is from 80 kΩ to 120 kΩ. The 80 kΩ value should be used for gain resistor calculation.

By varying the value of Rg it’s possible to match the span of AFE gain from Gain_min to Gain_max, see Figure 9. If there is no Rg value to match the desired range, then Rf value should be increased. For the current schematic Rf = 200 kΩ and Rg = 2.61 kΩ.

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Figure 9: Gain of AFE as a Function of Digital Rheostat's Code, Rgain = f(n)

Vref Divider Resistors Calculation

To calculate the values of DAC resistors (Rdiv1, RH0, Rdiv2 divider) the maximum range of Vref (Vcomp value) should be calculated. Considering the biggest possible gain of the AFE (Gain_max = 184.2) and the biggest possible input offset (see Figure 10):

the Vref can be changed by the value of Vcomp:

To find the value of Rdiv1, Rdiv2, the next equation system should be solved:

where RH0max – maximum resistance of the rheostat, in the worst case RH0max = 80 kΩ;

Van – is the voltage applied to the divider.

For the current schematic the nearest standard values of resistors are Rdiv2 = 75 kΩ, Rdiv1 = 46.4 kΩ.

Offset Error Sources and Offset Compensation

To set zero point for the AFE (zero pressure) the voltage from divider (Rdiv1, RH0, Rdiv2) is used. The output from the divider must be connected to the instrumentation amplifier through the buffer to eliminate the impact of DAC output resistance.

By changing the value of RH0 not only sensor offset, but OpAmps input offset voltages can be compensated. See the equation below.

Let’s add offset voltages to the equation (1):

where VosOpAmp0, VosOpAmp1, Vos_IntOpAmp – are input offset voltages of the SLG47004 amplifiers;

Vos_Input_Buffer – is buffer input offset voltage;

Vosbridge – is offset voltage of the sensor;

Vcm_error – is common-mode voltage error caused by inequality of internal R1, R2, R3, R4 resistors, and external Rf resistors. This voltage will be compensated after the trim procedure.

Vcomp – is the shift voltage from the divider for offset voltages compensation.

Note that signs of offset voltages were selected to show the worst-case error, see Figure 10.

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Figure 10: AFE with Offset Sources Placed to Show the Worst Case

Tuning Gain of AFE

Since the sensors span can be in the range from ±8.1 mV to ±9.2 mV for Van = 3.3 V, the gain of the instrumentation amplifier must be tuned to cover the full output range of the AFE.

The linear output swing of the SLG47004 OpAmps is from GND + 100 mV to VDD - 100 mV. It’s proposed to use the output range from VDDA*(32/64) to (VDDA - VDDA*(3/64)) for positive sensor output and from VDDA*(32/64) to VDDA*(3/64) for negative sensor output. VDDA*(3/64) is the threshold for internal Chopper ACMP, which is used for gain tuning. VDDA*(3/64) = 0.155 V for VDDA = 3.3 V.

Algorithm for Tuning Gain and Compensating Offset of the System

The initial value of RH0 and RH1 are 100 KΩ (80 KΩ in the worst case), code = 1024.

  • 1st step: offset compensation. Load the sensor with zero load (no load). Send to the SLG47004 I2C command to set the Virt Input0 (pulse to Set0 input of PT0 block) to logic High level. This will start the Auto-Trim procedure for RH0. Then I2C master should clear the Virt Input0 of the SLG47004, which is connected to Set0 input. During the Auto-Trim procedure the SLG47004 changes the value of RH0 until the output voltage of AFE reaches VDDA/2. After the end of the Auto‑Trim procedure (logic level of Idle/nActive output of PT block becomes High) the system is ready for the next step.
  • 2nd step: gain tuning. Load the sensor with a defined load. Send to the SLG47004 I2C command to set Virt Input1 (pulse to Set1 input of PT block) to logic High level. This will start the Auto-Trim procedure for RH1. Then I2C should clear the Virt Input1 of the SLG47004, which is connected to Set1 input. During this Auto-Trim procedure the SLG47004 changes the value of RH1 until the output voltage of AFE reaches (VDDA*(3/64)). After the end of the Auto-Trim procedure (logic level of Idle/Active output of PT block becomes High) the system is ready for the next step.
  • 3rd step: offset compensation. This step is the same as the 1st step.

Optionally, if higher accuracy is required, the User can add more offset/gain calibration steps considering the following limitations:

  • The Auto-Trim procedures of total offset compensation and system gain error must be done iteratively starting and finishing with the total offset compensation.
  • Total system offset (sensor offset + OpAmp1 offset + OpAmp2 offset) must not be greater than Vsensor_output_range/2.

Expected Gain errors after each tuning iteration are shown in Table 2.

Table 2: Expected Gain of AFE during Auto-Trim Procedure
Gain
Gain Error, %
Etalon gain
174.2
-
1st iteration (offset trim, then gain tuning)
170.9
1.89%
2nd iteration (offset trim, then gain tuning)
173.4
0.49%
3rd iteration (offset trim, then gain tuning)
174.4
0.09%

After the 3rd iteration the gain error is associated with the step error of digital rheostat.

Offset Compensation Accuracy

Assume that the Auto-Trim is done at temperature = 25 °C. The gain of the instrumentation amplifier is 273.3, RH resistance is 100 kΩ for code = 1024. Table 3 shows the accuracy of setting zero point (offset compensation).

Table 3: Accuracy of Setting Zero Point
Parameter
Value, V
Error in % of output sensor range
Step near set point (Vout[NRH0] – Vout[NRH0-1])
(Note 1)
0.0006
0.04 %
Step near set point considering rheostat DNL
(Note 2)
0.0012
0.08 %
Step error considering DNL and ACMP offset
(Note 3)
0.0015
0.1 %

Note 1    minimum achievable error of the Auto-Trim system is one trim step (± 1 of digital rheostat code, see Figure 11).

Note 2   Multiply ‘Step near set point’ value (Vout[NDR]- Vout[NDR-1]) by 2 (DNL error).

Note 3   Add the typical Chopper ACMP offset of 300 µV to the previous value.

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Figure 11: Error Sources of Offset Compensation Process

In the case of 10-bit ADC, the maximum error of the trimmed system is:

Please note that this error value is independent of Van voltage.

AFE with Internal Voltage Reference Source

Hardware Setup of AFE with Internal Voltage Reference

The SLG47004 allows powering all analog components of the AFE (sensor, DAC, and reference for Chopper ACMP) from an internal voltage source, see Figure 12. For this purpose, the SLG47004 has a special high drive buffer (HD Buffer macrocell).

P380#yIS1
Figure 12: Analog Front-End with Internal Voltage Reference

Precision Characteristics of Sensor at 2.048 V Supply Voltage

Characteristics of the pressure sensor at 2.048 V supply voltage are shown in Table 4. All other precision characteristics from Table 1 remain unchanged.

Table 4: Characteristics of Sensor at 2.048 V Supply Voltage
Parameter
Description
Value
Unit
ΔP
Pressure Range
±6.0
kPa
KVout
Output Voltage Span
±5.04 (min), ±5.32 (typ), ±5.73 (max)
mV
Kos_out
Null Offset
±0.154 (max)
mV
dVos_bridge/dT
Offset Temperature Drift (T = 0 to 50 °C)
±0.6 (max)
%FSS

Internal Macrocells Configurations

HD Buffer and OpAmp0 Vref Configurations

The HD Buffer shares the voltage reference with OpAmp0 Macrocell. Note that Vref can be connected (or disconnected) to OpAmp0 or HD Buffer macrocells independently. The configurations of HD Buffer and OpAmp0 Vref are shown in Figure 13.

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Figure 13: OpAmp Vref and HD Buffer Configurations

Chopper ACMP Configuration for AFE with 2.048 V Voltage Reference

Chopper ACMP configuration is shown in Figure 14.

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Figure 14: Chopper ACMP Configuration for AFE with 2.048 V Voltage Reference

Gain Resistor and DAC Divider Calculations for AFE with 2.048 V Voltage Reference

Considering the output span of the sensor (from 5.04 mV to 5.73 mV at Van = 2.048 V), the output voltage range of AFE must be

Using equation (3),

Gain resistor Rg = 1.33 kΩ, Rf = 100 kΩ. The range of offset compensation is Vcomp = ±(177.9*0.00215) = 0.382 V. The divider resistors Rdiv1 = 21.3 kΩ, Rdiv2 = 46.4 kΩ. The offset of the HD Buffer is 3 mV.

Software Simulation and Hardware Prototype Testing

Figure 15, Figure 16, Figure 17, and Figure 18 show the Auto-Trim process. Figure 15, Figure 16, and Figure 17 show the case when the duration of the pulse at Set input of RH is shorter than the duration of the Auto-Trim process. For this case, the stop condition for the Auto-Trim process is after the 2nd time there is a change at Up/Down input at the rising edge of the Clock input. Please refer to the datasheet to get more information about the Auto-Trim process.

If the User holds the Set input at high level, the Auto-Trim system will continue to operate and the output will follow the reference point, see Figure 18.

The maximum time of the first Auto-Trim iteration is RH_code/fAuto-Trim = 511/2048 = 250 ms.

Figure 15: Software Simulation Results of Offset Correction Process
Figure 16: Software Simulation Results of Offset Correction Process, Enlarged
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Figure 17: Auto-Trim Procedure with Short Pulse at Set Input of RH
P446#yIS1
Figure 18: Auto-Trim Procedure with Long Pulse at Set Input of RH

Conclusions

The application note describes the design procedure of the analog front-end for a Wheatstone bridge pressure sensor. A unique Auto-Trim feature of the SLG47004 is used to compensate for the offset of operational amplifiers and sensor, and to tune the gain of the AFE.

It was shown how to calculate gain and DAC resistors to cover the full output range and full trim range of the instrumentation amplifier.

To achieve the best precision, it's recommended to use iterative procedures of offset compensation and then gain tuning. The first and last procedure should be offset compensation. Practical results show that the best precision is achieved after the 3rd iteration. For the sensor and AFE described in this application note, the gain and offset errors after the Auto-Trim procedures are ≈0.1% of the sensor range.