TI中文支持网您好，关于6043的uss模块，PGA的输入范围我记得是35mv-1000mv，而给出的测试信号是包含y轴下半部分波形的，这个是因为在外部对信号进行了处理吗？还是uss模块设置后本身就能采集负电压？
Susan Yang：
请您以附件形式重新上传一下图片，谢谢
user5980769：
回复 Susan Yang:
这个怎么上传附件啊，其实和图没关系，就是MSP430FR6043 超声波感应评估模块 手册里给出的ADC结果有负值。一般来说换能器的接收原始信号不是+-多少mv吗，USS的ADC的采样输入范围是35mv-1000mv，我就想我就想问下这个负电压信号是怎么采进去的，是外部把电路处理把信号抬高了吗？还是uss中本身有这个功能？
Susan Yang：
回复 user5980769:
您指的是如下图所示？
在相关手册内有解释
The flow rates appear negative because air was pulled(instead of pushed) through the pipe.
user5980769：
回复 Susan Yang:
不是TOF，就是采集到的电压信号，很多文档里都有采集电压的图，就比如MSP430FR6043-based ultrasonic gas flow meter quick start guide文档中的图六，我就想知道他那个负采集电压是怎么采来的，是外部把电压抬高了？还是uss模块本身的功能？
Susan Yang：
回复 user5980769:
理解您的意思了
转换结果是有符号整数，范围为-2048 … + 2047。其零参考位于'FR6047内部产生的偏置电压。在每个模拟输入端使用1nF电容对偏置电压进行电气去耦。由于我们只对交流振幅感兴趣，因此不需要精确的偏置电压（~750mV）。所以使用有符号整数会更加简单一些
The first ADCs that came out had positive and negative supply voltage and operated with signed numbers already…. later when the first single supply ADC came out unsigned numbers were used. Some felt it is more convenient and it reflects the single supply feature…after all its just a convention….
We typically use an 3dB amplitude margin in the above case… therefore staying within +/- 1000 is a good capture signal.
user5980769：
回复 Susan Yang:
所以说要想采集换能器的有+-的信号需要先用外部电路将原始信号太高，将信号的0值抬到偏置电压，是这样吗？
调偏置的寄存器是PGABIAS吗？还有就是偏置都是750mv了，那USS采集的输入为信号电压范围是多少？
Susan Yang：
回复 user5980769:
是的，您的理解是正确的
常是用一个电阻器和cap完成的，如下图所示。
The below pictures are taken from the UG; The one just below shows the situation before the sonic pulse train arrives on CH1. The CH1_OUT terminal provides 0V/GND while CH1_IN terminal provides RxBias (which has the value of PgBias) via the switch. This charges up the coupling cap. But is also shorts the input of the input multiplexer to RxBias (no signal would be received at that moment).
Some microseconds before the acoustic signal is arriving the bias switch is opened; thus allowing the multiplexer to pick up the incoming pulse train. The coupling cap will hold the RxBias value long enough to provide the required DC-offset from ground…
