Test set up proposal to detect worst case

Most of the IEC61000-4-6 compliant generators do not offer the ability to generate a step smaller than 1 kHz. In order to determine the most critical noise frequency, it is usually required to use 100 Hz steps or even 10 Hz steps. The setup detailed in this section is an alternative to find out the most critical noise frequencies and to evaluate the robustness of the EUT at these frequencies.

In order to simulate a human finger, a copper coin (from10 to16 mm diameter) with a 500 Ω serial resistor connected to GND can be used.

The CDN adaptor is the same as the one used in IEC61000-4-6.

The following steps are needed to set the generator:

1. Select “sine wave” and “sweep” mode.
2. Sweep menu: time = 300 s, return time = 0, linear, interval = 1 ms
3. Set V_pp value on the generator to obtain the corresponding voltage injected on the EUT in case of standard test.

   Example: to test in the same condition as the standard at 3 V_rms, the user must adjust the generator voltage to inject 15 V_pp measured on the board with an oscilloscope.

   Note:

   On the AFG3102 Tektronix® generator, the modulation is not available in sweep mode.

4. Set the start and end frequencies such that the sweep range does not exceed 60 kHz for a 300 s duration. This recommendation allows the detection of the worst case level with sufficient accuracy (typically less than 5 % error).

The two following directions can be followed to improve noise immunity:

• Decrease noise level.
• Increase signal (measurement sensitivity).

The improvement techniques detailed below are based on hardware and software choices.

Several improvement techniques are introduced below:

• Active shield

  This feature increases the measurement sensitivity.

• Spread spectrum

  When activated, the spread spectrum creates several acquisition frequencies. This feature is particularly appropriate for reducing overall noise level inside the measurement when the noise is concentrated on certain frequencies (as opposed to a white noise).

• Detection thresholds

  Adjusting these parameters allows the optimization of the reported detection that is a compromise between false detection and detection loss.

• SW filter

  This feature, such as debounce filter, can be used to remove short and unwanted detections.

• Frequency hopping

  Upon detection of excessive noise on a dedicated channel, the firmware is able to change the acquisition frequency in order to move out of the disturbed frequency range. There may be some situations where frequency hopping and spread spectrum cannot be activated both at the same time. This is useful to get a Class B operation with no false touch detections (generally preferable than having some false touch detections).

• Channel blocking

  This feature uses an additional channel to detect noise and cancel touchkey detection if noise reaches a determined threshold. Sensors involved are ghost and guard-ring. Sensors involved are ghost and guard-ring.

• Impedance path to earth

  Decreasing the impedance path to earth is a good way to cancel the conducted noise effect (use of a metallic chassis, system ground and earth connected together). For instance, an application offering a direct connection between the earth and the system ground is not impacted by the conducted noise.

• Schottky diodes on sensor and shield

  Adding a low-capacitance Schottky diode like ST BAS70, increases the conducted-noise voltage level immunity. These Schottky diodes must be used on both sensors and shield.

• Reduce ripple on V_DD
• Sensor sensitivity

  Increasing sensor sensitivity decreases conducted-noise effects.

• Sampling capacitor C_s and serial resistor R_s

  Increasing C_s and/or R_s increases the acquisition time but decreases conducted-noise effects.

• Noise measurement on V_DD

  On application using the STM32 general purpose ADC, the LDO (or SMPS if any) input ripple can be measured and the application can decide to stop touch sensing acquisition when the conducted-noise voltage level is too high, or another counter measure such as increasing the debounce parameters.

The active shield is an electrode that wraps around the sensor. The goal is to minimize the parasitic capacitance between the sensor and the ground. To drive the shield electrode, a channel of a dedicated group with its own C_s can be used (see the figure below).

It is important to check the shied electrode waveform. C_sshield capacitance value and R_sshield serial resistor value must be adjusted in order to obtain the same signal shape as sensor electrode waveform (same amplitude, same rising and falling time).

The figure below shows the electrode sensor waveform in green and shield electrode waveform in yellow. The C_sshield is adjusted to obtain approximately the same charge level at the end of the acquisition.

The figure below is a zoom of the previous one at the beginning of acquisition: R_sshield and C_sshield are adjusted to obtain approximately the same rise and fall time on the shield electrode as sensor electrode waveform.

When the active shield is properly implemented, the count value is about twice as much as the count value without active shield. Since the noise level is not increased, the SNR is improved by a factor of 2, and noise immunity as well.

The negative impact of this feature is the requirement to dedicate two more I/Os and one more touch sensing group.

  htsc.Instance = TSC;
  …
  htsc.Init.ShieldIOs = TSC_GROUPx_IOy;
  htsc.Init.SamplingIOs = … |TSC_GROUPx_IOz|….;
  if (HAL_TSC_Init(&htsc) != HAL_OK)
  {
    Error_Handler();
  }

where TSC_GROUPx_IOy is the group of I/Os connected to R_sshield. (x = 1..8, y = 1..4) and TSC_GROUPx_IOz is the group of I/Os connected to C_sshield. (x = 1..8, z = 1..4 and z ≠ y).

/* Shield IOs definition */
#define SHIELD_IO_MSK      (TSC_GROUPx_IOy)

/* Bank(s) definition */
#define BANK_0_NBCHANNELS (2)
#define BANK_0_MSK_CHANNELS   (CHANNEL_0_IO_MSK | CHANNEL_1_IO_MSK | SHIELD_IO_MSK)
#define BANK_0_MSK_GROUPS     (CHANNEL_0_GRP_MSK | CHANNEL_1_GRP_MSK)

Example

In this example, the configuration is the one given in the figure below.

• Shield on group 1
  • G1_IO1 connect to C_sshield
  • G1_IO2 connect to R_sshield and shield copper
• Sensors on group 2
  • G2_IO1 connect to C_skey2
  • G2_IO2 connect to R_skey22 and sensor (Key22)
  • G2_IO3 connect to R_skey23 and sensor (Key23)
  • G2_IO4 connect to R_skey24 and sensor (Key24)
• Sensors on group 3
  • G3_IO1 connect to C_skey3
  • G3_IO2 connect to R_skey32 and sensor (Key32)
  • G3_IO3 connect to R_skey33 and sensor (Key33)
  • G3_IO4 connect to R_skey34 and sensor (Key34)
• Sensors on group 4
  • G4_IO1 connect to C_skey4
  • G4_IO2 connect to R_skey42 and sensor (Key42)
  • G4_IO3 connect to R_skey43 and sensor (Key43)
  • G4_IO4 connect to R_skey44 and sensor (Key44)

With this banks mapping, the waveforms on R_sxx and C_sxx are the ones given in the figures below.

To improve the conducted-noise immunity, the following R_s and C_s values give good performance:

• C_sshield = 100 nF (to adjust according to previous explanations in this section)
• R_sshield = 1 kΩ (to adjust according to previous explanations in this section)
• C_skeyx = 47 nF
• R_skeyxx = 10 kΩ