This is a common way of displaying channel noise in an analog system with a digitizing ADC. In this case, we short the two analog channel inputs (ECGP and ECGN) together and to VCM or letting each channel go to VMID using the input mux resources. In both cases, the two inputs are both held to a middle voltage within the linear input range of the input instrumentation amplifier – either both at VCM at 0.65V relative to GND or at VMID at 0.84V relative to GND when AVDD=1.8V. Then, several thousand measurements are taken with the ADC, and the raw ADC results are displayed in the histogram. The number of occurrences of each code is displayed on the Y-axis.
Ideally, the digitized code result histogram should be centered around 0. Instead, it is centered around -46 LSBs. So, there is an offset from zero. We are using an 18bit DAC, so there is one bit for polarity and 2^17 magnitude codes on each side. You can calculate the offset from the following formula:
ECG Reading (V) = (VREF×[18 bit ADC code]) / (2^17 × ECG Gain Setting)
Where VREF = 1V , ECG Gain Setting is programmable to 20, 40, 80, or 160 V/V .
The 18-bit ADC code is in two’s complement format so the right polarity needs to be considered when converting to decimal.
ECG Reading of -46 in Volts = (1V * (-46)) /((131072) * 20) = -17.548uV
ECG Reading of -47 in Voltage = (1V * (-47)) /((131072) * 20) = -17.929uV
The offset of the distribution below is currently somewhere between code -46 and code -47.
So, 100% of the 20V/V gain measurements of the ECG channel came in on codes -50, -49, -48, -47, -46, -45, -44, -42 and -41. Over half of the samples were digitized to either code -46 or code -47.