Sigma-delta ADC samples AC & DC signals synchronously

July 05, 2016 // By Graham Prophet
Analog Devices has recently published the datasheet of the AD7768-4, a 4-channel, simultaneous sampling Σ-Δ ADC with an Σ-Δ modulator and digital filter per channel, enabling synchronized sampling of both ac and dc signals. AD7768 is an 8-channel variant.

The device achieves 108 dB of dynamic range at a maximum input bandwidth of 110.8 kHz, combined with typical performance of ±2 ppm integral nonlinearity (INL), ±50 μV offset error, and ±30 ppm gain error. The AD7768-4 is available in 64-lead LQFP packaging, operates over the −40° C to +105° C industrial temperature range, and is priced at $10.75 (1000).

in 1000s.


The user can trade off input bandwidth, output data rate, and power dissipation, and select one of three power modes to optimize for noise targets and power consumption. The flexibility of the AD7768/AD7768-4 allows them to become reusable platforms for low power dc and high performance ac measurement modules. The three modes are; fast mode (256 ksample/sec maximum, 110.8 kHz input bandwidth, 51.5 mW per channel), median mode (128 ksample/sec maximum, 55.4 kHz input bandwidth, 27.5 mW per channel) and eco mode (32 ksample/sec maximum, 13.8 kHz input bandwidth, 9.375 mW per channel).


The conveters offer digital filtering capabilities, such as a wideband, low ±0.005 dB pass-band ripple, antialiasing low-pass filter with sharp roll-off, and 105 dB stop band attenuation at the Nyquist frequency. Frequency domain measurements can use the wideband linear phase filter. This filter has a flat pass band (±0.005 dB ripple) from dc to 102.4 kHz at 256 ksample/sec, from dc to 51.2 kHz at 128 ksample/sec, or from dc to 12.8 kHz at 32 ksample/sec.


The AD7768/AD7768-4 also offer sinc response via a sinc5 filter, a low latency path for low bandwidth, and low noise measurements. The wideband and sinc5 filters can be selected and run on a per channel basis. Within these filter options, the user can improve the dynamic range by selecting from decimation rates of ×32, ×64, ×128, ×256, ×512, and ×1024. The ability to vary the decimation filtering optimizes noise performance to the required input bandwidth.


Embedded analogue functionality on each ADC channel makes design easier,