The circuit described in this document provides an ultralow distortion driver circuit for the AD7995 10-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 AD7995 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 AD7995 has an I2C-compatible serial interface and is offered in an 8-lead SOT-23 package.
Figure 1: AD7995 ADC with the AD8599 Low Distortion Driver and AD780 Ultrahigh Precision Reference (Simplified Schematic: Decoupling and All Connections Not Shown)
It is always recommended to buffer analog input signals before applying them to ADCs with switched capacitor inputs such as the AD7995. 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 AD7995 can be operated as a 4-channel input device using VDD as a reference (the input voltage range is 0 V to VDD) or as a 3-channel input device with the fourth channel used as an external reference input, VREF (the input range is 0 V to VREF). 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 AD7995. A 1 μF decoupling capacitor is recommended on the VIN3 / VREF 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 AD7995 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 AD7995 input range from 0 V to +20 mV cannot be exercised (see MT-035 Tutorial).
The AD7995 can accept a reference input voltage from 1.2 V to VDD; therefore, different voltage reference sources can be used.