DESCRIPCIÓN GENERAL

The AD7788/AD7789 are low power, low noise, analog front ends for low frequency measurement applications. The AD7789 contains a low noise, 24-bit, ∑-∆ analog-to-digital converter (ADC) with one differential input. The AD7788 is a 16-bit version of the AD7789.
The devices operate from an internal clock. Therefore, the user does not have to supply a clock source to the devices. The output data rate is 16.6 Hz, which gives simultaneous 50 Hz/60 Hz rejection.
The devices operate with a single power supply from 2.5 V to 5.25 V. When operating from a 3 V supply, the power dissipation for the device is 225 µW maximum. The AD7788/AD7789 are available in a 10-lead MSOP.

 

CARACTERÍSTICAS

AD7788: 16-bit resolution
AD7789: 24-bit resolution
Supply: 2.5 V to 5.25 V operation
Normal: 75 µA maximum
Power-down: 1 µA maximum
RMS noise: 1.5 µV
AD7788: 16-bit p-p resolution
AD7789: 19-bit p-p resolution (21.5 bits effective)
Integral nonlinearity: 3.5 ppm typical
Simultaneous 50 Hz and 60 Hz rejection
Internal clock oscillator
VDD monitor channel
10-lead MSOP

 

INTERFACE

3-wire serial
SPI-, QSPI™-, MICROWIRE-, and DSP-compatible
Schmitt trigger on SCLK

 

APLICACIONES

Smart transmitters
Battery applications
Portable instrumentation
Sensor measurement
Temperature measurement
Pressure measurement
Weigh scales
4 to 20 mA loops

 

Más información

ADC CIRCUIT INFORMATION

The AD7788/AD7789 are low power ADCs that incorporate a Σ-Δ modulator and on-chip digital filtering intended for the measurement of wide dynamic range, low frequency signals, such as those in pressure transducers, weigh scales, and temperature measurement applications. The device has one unbuffered differential input. The device requires an external reference voltage between 0.1 V and VDD.

NOISE PERFORMANCE

Typically, the devices have an rms noise of 1.5 μV rms that corresponds to a peak-to-peak resolution of 16 bits for the AD7788 and 19 bits (equivalent to an effective resolution of 21.5 bits) for the AD7789. These numbers are for the bipolar input range with a reference of 2.5 V. The noise was measured with a differential input voltage of 0 V. The peak-to-peak resolution figures represent the resolution for which there is no code flicker within a six-sigma limit. The output noise comes from two sources. The first is the electrical noise in the semiconductor devices (device noise) used in the implementation of the modulator. The second is quantization noise, added when the analog input is converted into the digital domain.

DIGITAL INTERFACE

As previously outlined, the AD7788/AD7789 programmable functions are controlled using a set of on-chip registers. Data is written to these registers via the serial interface and read access to the on-chip registers is also provided by this interface. All communications with the devices must start with a write to the communications register. After power-on or reset, the devices expect a write to the communications register. The data written to this register determines whether the next operation is a readoperation or a write operation, and also determines to which register this read or write operation occurs. Therefore, write access to any of the other registers on the devices begins with a write operation to the communications register followed by a write to the selected register. A read operation from any other register (except when continuous read mode is selected) starts with a write to the communications register followed by a read operation from the selected register.

ANALOG INPUT CHANNEL

The AD7788/AD7789 have one differential analog input channel that is connected to the modulator, thus, the input is unbuffered. Note that this unbuffered input path provides a dynamic load to the driving source. Therefore, resistor/capacitor combinations on the input pins can cause dc gain errors, depending on the output impedance of the source that is driving the ADC input.