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AD8232 AC leads off detection?

Question asked by ivan87 on Feb 1, 2018

Dear Analog Team,

 

we're developing a wearable device and plan to use your ECG front-end. We have some issues while verifying the device and I kindly ask you to help clarify them.

 

The schematic is for 2 electrode measurement, RLD is connected to two 10MR resistors, the same as in the datasheet.  

 

We are verifying operation on four identical prototypes. All have AC leads off enabled (pin AC/DC is high). 

 

Q1) Why is there a DC offset between inputs when AC LOD is enabled? We measure (>10GOHM voltmeter) 0.5V with 7MR connected between inputs. All 4 modules show the same DC offset. 

 

Q2) Two of the four modules have IAOUT saturated at 3.3V. After we set AC/DC low the IAOUT goes back to 1.65V (reference voltage). When we connect a resistor decade to the input we see LOD+ active when 10MR is connected. As we decrease the resistance we first see LOD+ goes low (@9.5MR)..and couple of MR down (@7.3MR) we see IAOUT goes from 3.3 to Vref, i.e. 1.65V. 

 

Q3) If we put 10nF capacitor between the inputs the LOD+ is still high, how is this possible? This is a low impedance at 100kHz. 

 

All this is very odd, looks to us that AC LOD is not AC actually but DC which saturates the amplifier. This is bad as it saturates the amplifier if patient leads are briefly disconnected. 

 

Thank you for the help.

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