ADPD107 datasheet(47/66 Pages) AD

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Data Sheet

ADPD105/ADPD106/ADPD107

Rev. A | Page 47 of 66

Measuring Voltages Using the Current Inputs

In some situations, as in a case where a synchronized ECG and

PPG measurement is being attempted, a voltage measurement

must be made using the current inputs of the ADPD105/

ADPD106/ADPD107. First, convert the voltage source to a

current by placing a series resistor, RS, in series between the

voltage source and the current input of the device, as shown in

Figure 52.

TIA_VREF

TIA

ADC

RIN

IIN

VIN

ADPD105/ADPD106/

ADPD107

Figure 52. Using the ADPD105/ADPD106/ADPD107 Inputs with Voltage Sources

Secondly, there is a switch resistance, RIN, that must be taken

into consideration when converting a voltage source to a

current. This switch resistance is not a factor in a typical

photodiode application since the device is only dealing with

input currents and these currents are not a function of the input

resistance. However, when driving the device inputs from a

voltage source through a series resistance, RIN must be taken

into consideration. The conversion gain from a voltage into a

current can be determined by following the schematic in Figure 52.

IIN = (VIN − TIA_VREF)/(RS + RIN)

Typically, RIN is approximately 13 kΩ. The value of RIN varies as

a function of supply voltage. This variability is shown in Figure 53

1.70

1.75

1.80

1.85

1.90

1.95

VDD SUPPLY VOLTAGE (V)

–40°C

0°C

+25°C

+85°C

Figure 53. Variability of Switch Input Resistance as a Function of VDD Supply

Voltage

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