Using the AD7328 8-Channel ADC in Applications with Single-Ended Industrial-Level Signals
The AD7328 is designed on the iCMOS® (industrial CMOS) process. This is a process combining high voltage silicon with submicron CMOS and complementary bipolar technologies. The AD7328 can accept true bipolar analog input signals. The AD7328 has four software-selectable input ranges: ±10 V, ±5 V, ±2.5 V, and 0 V to +10 V. Each analog input channel can be independently programmed to one of the four input ranges. The analog input channels on the AD7328 can be programmed to be single ended, true differential, or pseudo differential.
Figure 1. Single-Ended-to-Differential Input (Simplified Schematic: Decoupling and All Connections Not Shown)
Differential operation requires that the VIN+ and VIN− be simultaneously driven with two signals of equal amplitude that are 180° out of phase. Because not all applications have a signal preconditioned for differential operation, there is often a need to perform a single-ended-to-differential conversion. This single-ended-to-differential conversion can be performed using an op-amp pair illustrated in Figure 1.
The AD8620 is an ideal choice of op amp that can be used to provide a single-ended-to-differential driver for AD7328. The AD8620 is a precision, low input bias current, wide bandwidth JFET dual operational amplifier.
The circuit configuration illustrated in Figure 1 shows how an AD8620 op amp can be used to convert a single-ended signal to a differential signal that can be applied to the AD7328 analog inputs. The signals at points VIN+ and VIN− must have equal amplitude and be 180° out of phase.
The circuit accepts a bipolar ±10 V single-ended signal. The dual AD8620 amplifiers are connected in a cross-coupled configuration to produce 20 V p-p signals at VIN+ and VIN− that are 180° out of phase. Therefore, the differential input signal to the AD7328, VDIFF, is 40 V p-p. The cross-coupled configuration provides for excel-lent phase match between the two outputs.
The AD7328 has a total of eight single-ended analog input channels. Figure 2 shows a typical connection diagram when operating the ADC in single-ended mode, where the AD797 is used to buffer the signal before applying it to the ADC analog inputs.
The analog input channels on the AD7328 can be independently programmed to accept one of four input ranges. The AD7328 can accept input signals of ±4 x VREFIN, ±2 x VREFIN, ±VREFIN, and 0 to 4 x VREFIN.
The AD7328 allows for an external reference voltage to be applied to the REFIN/REFOUT pin. The specified voltage input range on the reference voltage is from 2.5 V to 3 V. Using 3 V reference voltages instead of 2.5 V allows the AD7328 to accept larger input signals. In Figure 1 and Figure 2, the AD780 is used as an external reference source. The AD780 is a 2.5 V/3 V ultrahigh precision voltage reference, which allows flexibility in the voltage range selected.
To achieve the specified performance, 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 MT-031 Tutorial, MT-101 Tutorial, and the AD7328 evaluation board layout).
Figure 2. Single-Ended Input Operation (Simplified Schematic: Decoupling and All Connections Not Shown)