ADPD4200 datasheet(39/93 Pages) AD

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Data Sheet

ADPD4200

APPLICATIONS INFORMATION

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Rev. 0 | 39 of 93

mode, while the pulsed signal, with the ambient signal rejected, is

monitored in the time slot configured for measuring the desired LED

pulsed signal.

Protecting Against TIA Saturation in Normal

Operation

One of the reasons to monitor TIA ADC mode is to protect against

environments that can cause saturation. One concern when oper-

ating in high light conditions, especially with larger photodiodes,

is that the TIA stage can become saturated while the ADPD4200

continues to communicate data. The resulting saturation is not

typical. The TIA, based on its settings, can only handle a certain

level of photodiode current. Based on the way the ADPD4200 is

configured, if there is a current level from the photodiode that is

larger than the TIA can handle, the TIA output during the LED

pulse effectively extends the current pulse, making it wider. The

AFE timing is then violated because the positive portion of the BPF

output extends into the negative section of the integration window.

Thus, the photo signal is subtracted from itself, causing the output

signal to decrease when the effective light signal increases.

To measure the response from the TIA and verify that this stage

is not saturating, place the device in TIA ADC mode and slightly

modify the timing. Specifically, sweep INTEG_OFFSET_x until a

maximum is achieved. This procedure aligns the ADC sampling

time with the LED pulse to measure the total amount of light

falling on the photodetector (for example, background light and LED

pulse).

If this minimum value is below 16,384 LSBs, the TIA is not saturat-

ed. However, take care, because even if the result is not 16,384

LSBs, operating the device near saturation can quickly result in sat-

uration if light conditions change. A safe operating region is typically

at ¾ full scale and lower. The ADC resolution when operating in TIA

ADC mode with a buffer gain = 1 is shown in Table 23. These codes

are not the same as in modes with the BPF and integrator enabled

because the BPF and integrator are not unity-gain elements.

Table 23. ADC Resolution in TIA ADC Mode

TIA Gain (kΩ)

ADC Resolution (nA/LSB)

12.5

5.84

2.92

1.46

100

0.73

200

0.37

ECG MEASUREMENT WITH THE ADPD4200

The ADPD4200 can be used for ECG applications with the simple

addition of an external RC network as shown in Figure 38. The

electrical equivalent model for a dry electrode is shown along with

an RC circuit, external to the ADPD4200, consisting of two 500 kΩ

resistors in series with the inputs and a 470 pF capacitor across the

inputs, which serves as the sensing capacitor for the ECG signal.

The 500 kΩ resistors serve several purposes. For example, these

resistors provide short-circuit protection to limit the current that can

be injected into the body in the case of shorted input pins, to

increase overall input impedance of the ECG measurement, and to

improve common-mode rejection when there is an electrode imbal-

ance. The ECG signal is integrated onto the sensing capacitor. The

value of this capacitor is chosen such that an acceptable amount

of SNR is achieved, and the resultant RC time constant created

between the capacitor and the 200 kΩ resistor is such that at

least three time constants per sampling period are realized to fully

charge the capacitor.

Figure 38. Circuit for Measuring Single-Lead ECG with the ADPD4200

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