Design/Device suggestions

Hi Everyone,

I need to acquire 8 analog signals (+/-5Volts) with 16 bits resolution. Our G400D does not have enough resolution on its ADC to achieve this. There’s tons of ADC chip with 16 bits resolution but did’nt find one that would able to have positive and negative signals. This means we will need to design an interface board or find something already on the market. Of course deadline is not compliant with my schedule and (week number 38). Ideally I am looking for an end device (may be from automotive domain) that converts +/-5Volts on 8 inputs into CAN message (SAEJ1939)

Does anyone know where this graal is?

One of the issues I had recently was trying to find an ADC with ±5V input range with single supply at 3.3V. Seems this is not as easy as it sounds as many are single ended with the same input range as the supply or an external REF + and - which adds to the cost.

If your design is not critical you could consider what I did for a recent client and used 2 MCP3428 devices (I2C bus and 12, 16 or 18 bit precision) and if you connect them in differential mode you can get ±2.048V and use a potential divider on the input to allow ±5V. Don’t connect the -Ve input to ground on the supply but use this as the GND for the analog input. This way the device will read + and - voltages. Grounding the -Ve puts it into singled ended mode and positive values only.

I then did a simple 3 point calibration on each input and then the output tracks nicely with the input. I used 0.1% resistors for the divider network with as low a temperature coefficient as I could find.

The other way is to use an op-amp but this means having to use positive and negative supplies. My design above will work with a single 3.3 or 5.0V supply.

You can create a negative voltage rail with this simple trick EEVblog #483 - Microcontroller Voltage Inverter Tutorial - YouTube

And my favourite trick for increasing the effective number of bits is oversampling.

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There should be dozens of 16-24 bit, bipolar or differential input (will accept positive and negative voltage inputs) ADCs assuming you’re sampling at reasonably low rates like less than 1 Msample/second. Check out the parametric IC selector at Linear Technology or Analog Devices and make sure you select the bipolar/unipolar column and the power supply range column.|18|24

TI has a broad range of ADCs also. The nice thing about all 3 of these companies is they provide relatively inexpensive evaluation modules for almost all their ADCs that allow you to evaluate their performance on your signals very easily. The evaluation modules also provide documentation with schematics, parts lists and even PCB layout files as a reference design.

Unfortunately, there are a lot of issues you need to get right to pick the right ADC. If you can be a little more specific about things like sample rate, actual required dynamic range, anti-aliasing requirements, single or bipolar power, the nature of the signals you’re sampling like output impedance and differential/single ended. I could maybe be a little more specific in recommending an ADC

I love Linear devices even though they are expensive but why do all their datasheets and app notes used non standard resistor values. I can see this for certain things like feedback but a 10.2K resistor for a pullup is going a bit too far :slight_smile:

@ Dave McLaughlin - I agree, 10.2K is kind of a weird value for a pull up. It is a standard value and readily available at Digikey but still kind of weird. Probably the engineer designing the reference circuit had a bunch laying around her bench so that is what she used and no one has thought about it since. Until you :slight_smile:

[url][/url] + pulse counters?

I had to read -2.5v to +2.5v signals on a G30. Used an Mcp3208 and an SI8641BD-B-IS. Isolated the second power rails.