The circuit described in this document provides an ultralow distortion driver circuit for the AD7991 12-bit, 4-channel ADC, which is designed to achieve optimum ac and dc performance. The circuit uses the ultralow distortion, ultralow noise AD8599 dual-supply op amp and ultrahigh precision AD780 band gap voltage reference to ensure that the maximum AD7991 performance is achieved by providing a low impedance driver with adequate settling time and a highly accurate reference voltage. The AD8599 is a dual operational amplifier that operates with supplies from ±4.5 V to ±18 V. The AD7991 has an I²C-compatible serial interface and is offered in an 8-lead SOT-23 package.

It is always recommended to buffer analog input signals before applying them to ADCs with switched capacitor inputs such as the AD7991. This is particularly important in applications where the signal source has high source impedance and where low distortion and high signal-to-noise-ratio is important. The circuit illustrated in Figure 1 shows how the AD8599, an ideal choice for high accuracy designs, can be used to buffer the analog input channels.

The AD7991 can be operated as a 4-channel input device using V_DD as a reference (the input voltage range is 0 V to V_DD) or as a 3-channel input device with the fourth channel used as an external reference input, V_REF (the input range is 0 V to V_REF). These options are programmable via the I²C-compatible interface. The AD780 is a 2.5 V/3 V ultrahigh precision, band gap voltage reference and is recommended for use with the AD7991. A 1 μF decoupling capacitor is recommended on the V_IN3/V_REF signal for best performance.

Care must be taken 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. Each diode can conduct a maximum current of 10 mA without causing irreversible damage to the part. The MT-036 Tutorial discusses methods to protect the input circuits of op amps and ADCs against such damage.

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 MT-031 Tutorial, MT-101 Tutorial, and the AD7991 evaluation board layout).

If single-supply op amp operation is required for buffering the input signal, the AD8605 is a suitable choice. Note that the output of the AD8605 operating on a single +5 V supply can only go to approximately 20 mV above ground; therefore, the AD7991 input range from 0 V to +20 mV cannot be exercised (see MT-035 Tutorial).

The AD7991 can accept a reference input voltage from 1.2 V to V_DD; therefore, different voltage reference sources can be used.