The table provides the SNR, ENOB, and amount of noise generated for five typical ADCs, each with a 1.024-V input range. It also shows what the addition of external noise does to the effective resolution. For example, adding 100 µV of noise limits performance to a little better than 11 bits. There are three easy methods for limiting external noise.
BUFFER THE CONNECTION
You buy a 12-bit, 1-Msample/s ADC and connect it to your data bus. You quickly find out that you don’t have anything close to 12 bits of resolution. Ideally, when the ADC’s output is disabled, it is isolated from the data bus. As shown in the left half of Figure 2, the area below the high impedance pins capacitively couples the bus noise to the ADC’s substrate, reducing the overall SNR.
Look at your processor’s data bus with a high-speed oscilloscope and you will see what an awful noise generator it is. This can be fixed with the addition of digital buffers to isolate the ADC from the data bus. As shown in the right half of Figure 2, the bus noise is coupled to the digital buffer’s substrate.
Some engineers balk at the idea of spending an extra 30 cents in buffers for their six-dollar ADC. (When put in these terms, it obviously seems so very short sighted.) If cost is an issue, then an 8-bit ADC would even be a lot cheaper, although it also wouldn’t have a chance of meeting the effective resolution requirement.
If the sample rate is not too high, serial ADCs can be used. The benefit is a smaller package, fewer pins, and no connection to a noisy bus. The disadvantage is that it will require some sort of serial interface to your microcontroller.
SHIELD OR GUARD ANALOG SIGNALS
Ideally, digital signals should be kept away from your analog signals, but sometimes this isn’t possible. Figure 3 shows how a digital signal will capacitively couple its edges to an analog signal. The longer and closer they run together, the greater the coupling. If lines must be run together, put a grounded line between them (Fig. 3b). With a shield, any digital noise is shunted to ground. The disadvantage is that with the shield being so close, the analog signal has an increased distributed capacitance, inadvertently making a low-pass filter. Using a guard solves this (Fig. 3c).
The input of the buffer is connected to the analog signal while the output is connected to what was the shield. The digital noise is still shunted away to the low impedance of the output buffer. However, with the guard being at the same potential as the analog signal, no current flows through the coupling capacitance and no filter is created.
AUTO ROUTING
Auto routing is the curse of high-resolution ADC systems. If your system is greater than 12 bits, turn it off. At one company where I routinely laid out boards for 20 bits, I specified that the board software we were to purchase must not include the auto router. When one supplier offered to sweeten the pot by including it for free, I told him I would cancel the order if he did. If that is not an option, hand-route the sensitive analog sections before turning to the auto router.
any more little tricks can help reduce the amount of user contributed noise. I will discuss more of them in future columns.