CN0247

To maximize performance, most SAR ADCs require a suitable input buffer for the analog signal. The buffer isolates the source from the transients generated on the ADC input when the internal track-and-hold switch goes from hold to track. The buffer driving the ADC must recover from this transient and settle to the required accuracy within the acquisition time of the ADC. This is particularly important in applications where the signal source has high source impedance and where low distortion and a high signal-to-noise ratio is important. Selecting the proper buffer op amp is therefore a critical part of the design process.

The AD7091R is a 12-bit, fast, ultralow power, single-supply ADC with an internal 2.5 V reference. The part can be operated from a 2.7 V to 5.25 V supply. The AD7091R is capable of throughput rates of 1 MSPS. At a sampling rate of 1 MSPS, the total power dissipation of the AD7091R with a 10 kHz input signal is approximately 2.3 mW.

This power figure can be reduced in applications where 1 MSPS sampling frequency is not necessary because the AD7091R power scales with the throughput, as shown in Table 1.

To further reduce power dissipation of the system, reduce the throughput rate of the converter. Table 1 shows the typical power vs. throughput for the AD7091R at 3 V when operating in normal mode.

Table 1 shows the reduction in power consumption that is achievable when power-down mode is activated. The powerdown mode is an extremely useful method to significantly reduce power supply requirements when operating the AD7091R at lower throughput rates.

The AD7091R is housed in a tiny, 3 mm × 2 mm, 10-lead LFCSP or a 3 mm × 5 mm, 10-lead MSOP. Both packages offer considerable space saving advantages over competitive solutions.

The AD8031 is a rail-to-rail input/output low power operational amplifier and is an optimum drive amplifier for the AD7091R. The AD8031 can operate from a 2.7 V to 12 V supply, which allows both ICs to be driven from the same supply rail. The AD8031 has an 80 MHz bandwidth, a 30 V/μs slew rate, and a 125 ns settling time to 0.1%.

When operating on a single supply, the output of the AD8031 can go to within 20 mV of the negative rail. If linearity to 0 V input is needed, the AD8031 requires an additional negative supply (see Tutorial MT-035).

Figure 1 shows the simplified schematic for the circuit. Well decouple the IC power supply pins to ground using 100 nF and 10 μF ceramic capacitors. Position these capacitors as close as possible to the supply pins of both ICs.

Take care to ensure that the analog input signal to the ADC does not exceed the supply rails by more than 300 mV. If the signal does exceed this level, the internal ESD protection diodes become forward-biased and start conducting current into the substrate. A diode can conduct a maximum current of 10 mA without causing irreversible damage to the part. This can be prevented by connecting a pair of Schottky diodes between VIN and the supply rails of the AD7091R, as described in Tutorial MT-036).

The AD7091R contains an internal 2.5 V reference. Well decouple the REF_IN/REF_OUT pin to achieve the specified performance. The typical value for the REF_IN/REF_OUT capacitor is 2.2 μF. Note that the internal reference voltage can be overdriven externally.

If an external reference voltage is used, the range must be between 2.7 V to V_DD, and it must be connected to the REF_IN/REF_OUT pin. A typical value for the regulator bypass (REGCAP) decoupling capacitor is 1 μF.

The voltage applied to the V_DRIVE input controls the logic level voltage of the serial interface. Connect this pin to the supply voltage of the logic family connected to the AD7091R digital outputs. V_DRIVE can be set in the 1.8 V to V_DD range. Typical values for the V_DRIVE decoupling capacitors are 100 nF in parallel with 10 μF.

If the busy indication function is required, connect a pull-up resistor of 100 kΩ between the V_DRIVE and the SDO pin.

The AD8031 that is used to buffer analog input of the AD7091R is configured as a unity gain buffer. A single-pole RC filter follows the op amp output stage to reduce out-of-band noise. The cutoff frequency of the RC filter is set to 660 kHz. However, depending on the system throughput rate specification, this parameter can vary. In systems where the AD7091R is not operated at maximum throughput rate, the filter cutoff frequency can be reduced. Depending on the analog signal input amplitude and offset, the AD8031 operational amplifier can be configured to provide gain, attenuation, and level shifting, to match the input signal swing to the analog input range of the ADC.

Note that convert start pulse width = 20 ns when sampling, V_DD = V_DRIVE = 3 V.

Figure 2 and Figure 3 show the integral nonlinearity (INL) and differential nonlinearity (DNL) plots for the circuit. Note that the INL and DNL is less than ±1 LSB.

Figure 4 displays the FFT data calculated for 8192 samples captured at the 1 MSPS rate with an analog input frequency of 10 kHz. The SNR is 70.44 dBFS.

The circuit must be constructed on a multilayer printed circuit board (PCB) with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see Tutorial MT-031, Tutorial MT-101, and the AD7091R evaluation board layout shown in the CN-0247 Design Support Package.

Values of the components surrounding the AD7091R and the AD8031 can be modified to meet specific requirements of the application and sensor. For example, the buffer can be configured to provide gain and offset, and the cutoff frequency of the RC filter can be changed depending on the sampling frequency and input frequency.

A complete documentation package including schematics, board layout, and bill of materials (BOM) can be found at http://www.analog.com/CN0247-DesignSupport.