K60 ADC, Differential without PGA, VREFH vs VREF_OUT

[pmt]

I am using the K60 16-bit ADC converters similar to what is described in AN-4568.

I am using this in differential mode without PGA.

When PGA is used it is clear that the VREF_OUT signal is required and the governing voltage reference for the measurement. However when PGA is not used [refer to Table 2, ADC16 MEASUREMENT (PGA DISABLED)] it not clear what effect VREF_OUT or VREFH play in the ADC conversion.

Seemingly VREF_OUT is NOT needed in differential PGA disabled mode, yet the differential input range is limited to 2400mV. But where does this come from if VREF_OUT can be disabled? What effect, if any does VREFH input play in the measurement in this mode?

Thanks,

PMT

[xiangjun_rong]

 Hi, Pmt

The reference voltage of the PGA module is VREF_OUT, which is 1.2V. You can connect an external  1.2V voltage source to VREF_OUT pin or configure the VREF module so that it can output 1.2V for VREF_OUT pin.

The reference voltage of  ADC16 module can be from VREF_OUT or VREFH pin, which is selected by the ADCx_SC2[REFSEL],if the
ADCx_SC2[REFSEL]=00, the 3.3V voltage reference from(Vrefh,Vrefl) is selected. If
ADCx_SC2[REFSEL]=01, the Vref_OUT 1.2V voltage reference is selected for ADC16 module.

If you use PGA, generally, we suggest you use 1.2V Vref_OUT as voltage reference of ADC16, but you can still use 3.3V voltage reference from VREFH pin for ADC16 module.

If you do not use PGA, in general, we suggest you use 3.3V voltage reference from VREFH pin, but you can still use VREF_OUT 1.2V as voltage reference of ADC16.

If you use 3.3V voltage reference, the maximum ADC input voltage is 3.3V in single-ended mode, in differential mode, it is -3.3V~+3.3V, the Vpp is 6.6V. If you use 1.2V voltage reference, the maximum ADC input voltage is 1.2V in single-ended mode, in differential mode, it is -1.2V~+1.2V, the Vpp is 2.4V.

The AN4568 use 1.2V voltage reference for ADC16.

Hope it can help you