Features: Serial Interface; Guaranteed No Missing Codes; 20kHz Sampling Rate; Low Supply Current: 250µA;

Applications: remote data acquisition; stand-alone data acquisition; sensor interface; battery powered systems;

Electrostatic Discharge Sensitivity

Electrostatic discharge can cause damage ranging from performance degradation to complete equipment failure. Burrbrown Corporation recommends handling and storage of all integrated circuits with appropriate ESD protection methods.

ESD damage can range from minor performance degradation to complete device failure. Precision integrated circuits can be more susceptible to damage because very small parameter changes can cause the device to not meet published specifications.

serial interface

The ADS1286 communicates with microprocessors and other external digital systems through a synchronous 3-wire serial interface. The dclock synchronizes the data transfer with each bit transmitted on the falling dclock edge and captures those bits, making the dclock run to clock the lsb data or zero first. If the CS input is not running rail-to-rail, the input logic buffer will sink current. This current can be large compared to typical supply current. To get the lowest supply current, connect the CS pin to ground when it is low and power it when it is high.

Rising dclock edge in the receiving system. Falling CS initiates data transfer, as shown in Figure 1. After CS falls, the second dclock pulse enables dout. After a vacant bit, the A/D conversion result is output on the output line. Bring CS High to reset the ADS1286 for the next data exchange.

Micropower operation

With a typical operating current of 250 microamps and automatic shutdown between conversions, the ADS1286 achieves extremely low power consumption over a wide sample rate range (see Figure 2). Automatic shutdown allows supply current to decrease with sampling rate.

shutdown

The ADS1286 is equipped with an automatic shutdown FEA-TUS. When the CS pin is low, the device draws power, and when the pin is high, the device shuts down completely. The bias circuits and comparators are powered down and the reference input goes high impedance at the end of each conversion to let the dclock run to clock the lsb first data or zero. If the CS input is not running rail-to-rail, the input logic buffer will sink current. This current can be large compared to typical supply current. For the lowest supply current, connect the CS pin to ground at low currents and power it at high currents.

Minimize power consumption

In systems with long transition intervals, the lowest power consumption will occur when the minimum cs time is short. Turning cs down to transfer data as fast as possible, then turning it up again results in the lowest current draw. This minimizes the amount of time the device consumes power. After A/D auto-conversion - caly is off even if cs is held low. The logic will draw a small amount of current if the clock is left running for LSB data or zero clock output (see Figure 3).

Simplified reference operations

The effective resolution of the ADS1286 can be increased by reducing the input range of the converter. The ADS1286 exhibits good linearity and gain over a wide reference voltage range (see the typical performance curves "Variation of Linearity vs. Reference Voltage" and "Gain Variation vs. Reference Voltage"). However, operation at low VREF values due to the reduced LSB must be careful.

rc input filter

The input can be filtered with an RC network, as shown in Figure 4. For large values of cfilter (eg, 1 μf), the capacitive input switch current is averaged to a net DC current. Therefore, a filter with small resistance and large capacitance should be selected to prevent the DC voltage drop through the resistance. The magnitude of the DC current is about idc=20pf x vin/tcyc, which is roughly proportional to vin. When running with a minimum cycle time of 64 microseconds, the input current is equal to 1.56 microamps at vin=5v. In this case, a 75Ω filter resistor will result in a full-scale error of 0.1lsb. If a larger filter resistor must be used, the error can be eliminated by increasing the cycle time.

size and thus the higher precision requirements placed on the converter. When operating at low VREF values, the following factors must be considered: 1. cancellation; 2. noise.

Decrease the offset of VREF

The offset of the ADS1286 has a greater impact on the output code. when the adc works with a lower reference voltage. As the size of the LSB decreases, the offset (usually a fixed voltage) becomes a larger fraction of the LSB. The typical performance curve "Offset vs. Reference Voltage" shows the relationship between the offset in the LSB and the reference voltage for a typical value of VOS. For example, a VOS of 122 μV of 0.1LSB with a 5V reference becomes 0.5LSB with a 1V reference and 2.5LSB with a 0.2V reference. If this offset is not acceptable, it can be digitally corrected by the receiving system or by canceling the negative input of the ADS1286.

Reduce the noise of VREF

Using ground planes, good bypassing, good layout techniques, and minimizing noise on the referenced input, the total input-referred noise of the ads1286 can be reduced to about 200µV peak-to-peak. For a 5V reference, this noise is insignificant, but as the size of the LSB decreases, it becomes a larger fraction of the LSB.

For operation using a 5V reference, the 200µV noise is only 0.15LSB peak-to-peak. In this case, the ADS1286 noise has little uncertainty in the output code. However, with a reduced reference, noise can become a significant part of the lsb and cause undesired jitter in the output code. For example, the same 200µV noise is 0.3LSB peak-to-peak when the reference voltage is 2.5V. If the reference is further lowered to 1v, the 200µv noise becomes equal to 0.8lsb and stable code may be difficult to achieve. In this case, multiple readings may need to be averaged.