This circuit is designed to optimize the performance of the AD7328 bipolar input, 8-channel, 12-bit plus sign ADC. The AD7328 can operate at a throughput rate of 1 MSPS. The selected operational amplifier (op amp) and reference voltage source ensure that maximum AD7328 performance is achieved with industrial-level, single-ended signal sources by providing a low impedance driver with adequate settling time and an accurate reference supply.

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.

In applications where harmonic distortion and signal-to-noise ratio are critical specifications, the analog input of the AD7328 should be driven from a low impedance source. Large source impedances significantly affect the ac performance of the ADC and can necessitate the use of an input buffer amplifier. When no amplifier is used to drive the analog input, the source impedance should be limited to low values. Due to the programmable nature of the analog inputs on the AD7328, the choice of op amp used to drive the inputs is a function of the particular application and depends on the input configuration and the analog input voltage ranges selected.

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, V_DIFF, 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 V_REFIN, ±2 x V_REFIN, ±V_REFIN, and 0 to 4 x V_REFIN.

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).

Suitable voltage references for the AD7328 include the REF192, AD1582, ADR03, ADR381, ADR391, and ADR421. The AD8022 dual high-speed, low-noise op-amp can also be used in high-frequency applications where a dual op-amp is desired. In high-performance systems, a pair of AD8021s, a single-channel variant of the AD8022 can also be used in place of the AD8022. For lower frequency, single-ended applications, op-amps such as AD797 (single) and AD8610 (single), AD8620 (dual), AD8599 (dual), and ADA4941-1 (differential), are suitable alternatives.