AD7265 12-Bit, 3-Channel SAR ADC in Differential and Single-Ended Configurations Using the AD8022 High Speed Op Amp
Figure 1. AD8022 DC-Coupled Circuit to Convert a Bipolar Single-Ended Signal into a Unipolar Differential Signal (Simplified Schematic: Decoupling and All Connections Not Shown)
The circuit configurations illustrated in Figure 1 shows how an AD8022 op amp can be used to convert a bipolar single-ended signal into a unipolar differential signal that can be applied directly to the AD7265 analog inputs. The circuit not only performs the single-ended-to-differential conversion but also level shifts the output signal to match the ADC input range. The voltage applied to Point A sets up the common-mode voltage for each half of the AD8022. The 10 kΩ/20 kΩ divider generates this voltage (1.67 V) from the AD7265 2.5 V internal reference. If the on-chip 2.5 V reference on the AD7265 is to be used elsewhere in a system (as illustrated in Figure 1and Figure 2), the output from DCAPA and DCAPB must first be buffered. The OP177 features the highest precision performance of any op amp currently available and is a perfect choice for a reference buffer.
The primary negative feedback path is provided by R2 to R1, and the gain from VIN to VA2 is set by the ratio of R2 to R1. In this case, the ratio is 0.5. The common-mode voltage of 1.67 V at the input of the upper half of the AD8022 produces an output common-mode voltage at VA2 of (1 + R2/R1) × 1.67 V = 2.5 V. Localized feedback supplied by the R3 and R4 resistors produces a signal at VA1 that is 180° out of phase with the signal at VA2.
Figure 2. AD8022 DC-Coupled Circuit for Single-Ended Input Mode of Operation (Simplified Schematic: Decoupling and All Connections Not Shown)
When the input voltage is 0 V, VA1 and VA2 must be 2.5 V. This requires a current in both R3 and R4 of (2.5 V – 1.67 V)/221 Ω = 3.76 mA. The current through R5 is therefore 2 × 3.76 mA = 7.52 mA. Hence, R5 must be equal to R3 and R4 to force the common-mode voltage at VA1 to be 2.5 V.
The AD7265 can have a total of 12 single-ended analog input channels. The analog input range can be programmed to be either 0 V to VREF or 0 V to 2 × VREF . Figure 2 shows a typical connection diagram when operating the ADC in single-ended mode, where an AD8022 is used to drive a pair of discrete channels. The AD8021 is a high performance single op amp that can be used as an alternative to a dual device in very high performance systems. The absolute value of R is flexible, but it must be chosen to achieve the desired bandwidth of the op amp.
In both Figure 1 and Figure 2, the AD8022 operates on dual 12 V supplies while the AD7265 is specified for power supply voltages of 2.7 V to 5.25 V. Care must be taken to ensure that the input maximum input voltage limits of the AD7265are not exceeded during transient or power-on conditions (see the MT-036 Tutorial). In addition, the circuit must be constructed on a multilayer PC board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see the MT-031 Tutorial, the MT-101 Tutorial, and the AD7265 evaluation board layout).