The op amp chosen for this application should have a low offset voltage to minimize the resulting ADC gain and offset errors. The LMC6082A was chosen for its low offset voltage (800 µV maximum, over temperature) and reasonable price ($2.53 in hundred-unit quantities). The 0.01-µF op-amp feedback capacitor adds a low-frequency pole to the op-amp transfer function, ensuring circuit stability (Fig. 3). The bypass capacitors at the ADC's reference pins provide a low-impedance ac path to ground for the top and bottom of the reference ladder, keeping the reference pins quiet.
The reference driver circuit in Figure 4 is intended for ADCs without reference force and sense pins. This circuit uses a low-impedance buffer with the feedback loop closed around it. The 0.1-µF capacitor in the op-amp feedback loop ensures stability by reducing the high-frequency gain to unity. The output transistors are configured as emitter followers (high current gain, unity voltage gain). The additional voltage gain of a common-source or common-emitter configuration in the feedback loop of an op amp invites instability. That's why those configurations aren't recommended.
There are many ways to connect the analog and digital grounds of an ADC. The most important thing to remember is that they should both remain at the same potential at all times. One of the oldest grounding methods is to use separate analog and digital ground traces from the ADC to the power supply or to the board's power-ground entry point. The difficulty in doing this, though, is that the magnitude and frequency components of the analog and digital currents differ. This results in a potential difference between the analog and digital grounds of the converter as these currents flow through their respective trace resistances to a common point (Fig. 5). Another big problem with this technique is that trial and error, lots of experience, and a bit of luck are needed to secure low noise levels.
Alternative Ground Techniques
Also, grounding can be accomplished by connecting all ground pins to the analog ground plane at the ADC. This can work well for ADCs with relatively low-frequency digital ground currents that wouldn't add significant noise to the analog ground plane. But it isn't desirable for high-speed ADCs, because their fast logic edge rates can add digital noise to the analog ground plane. Avoid this technique for sample rates over a few hundred kilosamples per second.
For very good first-time results, use separate analog and digital ground planes placed in the same board layer. The boundary between these planes should pass beneath the ADC and separate the pins with analog functions from those with digital functions. The two ground planes should be connected beneath the ADC by a narrow trace, the width of which is a function of the copper thickness, the analog ground-current frequency, and the amount of analog current going through this point.
We have empirically found that a width of 2 to 3 cm will work well with 1-oz. copper and FR-4 board material. This narrow connection provides a relatively high impedance to the flow of high-frequency digital ground currents with high-edge rates, but a relatively low impedance to the analog ground currents with lower-frequency components. A suggested ground plane layout for the ADC10030, a 10-bit, 27-Msample/s ADC, is shown in Figure 6.
Compared to the digital ground currents, the analog energy can flow through the connection between the analog and digital ground planes more easily. Consequently, the power-supply ground should be connected to the digital ground plane.
To avoid interaction between the ADC ground current and the digital ground currents of any high-powered digital components, locate the ADC's linear power-supply ground or its regulator close to the ADC. Place the switching regulator and any high-current digital components as far from the ADC as possible.
On the other hand, if you must use a switching supply without a linear regulator, filter the ADC supply voltages well and locate the switching supply as far as practically possible from the ADC. The ADC ground current and high-power digital ground-current paths should not run in parallel with each other. They also should be kept as physically far as practically possible from each other.
Figures 7a and 7b show examples of poor layouts. If the ADC and high-power digital ground-return paths are common, the high-power digital ground-current fluctuations can cause voltage fluctuations in the ground path that are seen at the ADC. These ground voltage fluctuations appear in the ADC input as noise. Figure 7c shows better positioning of these components.
The analog and digital ground planes should not overlap each other. Current in the digital ground plane may couple energy into the analog ground plane, adding noise to the analog circuitry. Since the grounds don't overlap each other, it makes sense to use a single layer for the analog and the digital ground planes. Digital noise can be coupled into the analog circuitry if digital components are placed over the analog ground plane, or vice versa. Place analog components over the analog ground plane and digital components over the digital ground plane to keep digital noise out of the analog circuitry.
You'll get the best high-frequency performance with a physically straight signal path. A path that folds back upon itself can lead to capacitive and inductive coupling that can cause unwanted feedback, resulting in increased distortion and noise. The best layout is one that produces a straight or nearly straight signal path. Also, be especially careful with inductors. Mutual inductance can change the characteristics of the circuit in which it is used. Orient inductors 90° to each other with a minimum separation equal to the length of their bodies. Alternatively, place them in line with each other, again separated by at least the length of their bodies. With high current levels, additional separation may be required.
It's not too difficult to obtain the high data-sheet performance touted for some high-speed ADCs. But it does require knowledge and application of sound design and layout rules. Proper attention to power-supply connections, voltage reference considerations, and layout and grounding go a long way toward getting there.