在AFE4400使用中遇到的问题

最近再用AFE4400测血氧，始终不准确。

看了社区里说可以这样检查AFE4400电路是否正常：

1. 0.75V on TX_REF pin (pin#9)

2. 1V on BG pin (pin#7)

3. 0.9V on VCM pin (pin#4)

4. 4MHz clock on CLKOUT pin (pin#30) - assuming there is an external 8MHz crystal on XIN-XOUT pins

我在检查的时候发现：TX_REF始终在3.3V，其他的几项都符合要求。

这意味着什么？

The RED and IR LED reference currents can be independently set. The current source (ILED) locally regulates and ensures that the actual LED current tracks the specified reference. The transmitter section uses an internal 0.5-V reference voltage for operation. This reference voltage is available on the REF_TX pin and must be decoupled to ground with a 2.2-μF capacitor. The TX_REF voltage is derived from the TX_CTRL_SUP. The maximum LED current setting supports up to 50-mA LED current.

TX_REF is the voltage that governs the generation of the LED current from the internal reference voltage. Choosing the lowest allowed TX_REF setting reduces the additional headroom required but results in higher transmitter noise. Other than for the highest-end clinical SPO2 applications, this extra noise resulting from a lower TX_REF setting can be acceptable.

这是个 LED 控制电流的功能.