Input | ADC Input | Digital Output ADS7056 |
---|---|---|
VinMin = 0V | AIN_P = 0V, AIN_M = 0V | 000H or 010 |
VinMax = 10V | AIN_P = 3.3V, AIN_M = 0V | FFFH or 409610 |
Vcc | AVDD | DVDD |
---|---|---|
12V | 3V | 1.65V |
This design is meant for increases the input range of a low-power SAR ADC by attenuating the input signal to match the full-scale range. The values in the component selection section can be adjusted to allow for a different input voltage range on the amplifier and full-scale range on the ADC. The input signal in this design is first buffered by the OPA192 device, a high-voltage precision amplifier, to avoid gain errors from the signal source impedance. A resistor divider at the output of the OPA192 device is then used to attenuate the signal and is again buffered by the OPA365 device. This wide-bandwidth amplifier allows the ADS7056 device to be used at a max sampling rate of 2.5MSPS. This circuit implementation is applicable to test and measurement, appliances, and factory automation and control. In general, this circuit can be used for most applications where a higher voltage signal needs to interface with a low-voltage single-ended ADC.
Specification | Goal | Calculated | Simulated |
---|---|---|---|
Transient ADC Input Settling | < 0.5 × LSB = 91.5µV | 16µV | |
Bandwidth | > 5MHz | 8.09MHz | 7.04MHz |
Noise | < 0.5 × LSB | 20.85µVRMS | 22.74µVRMS |
The following graph shows a linear output response for inputs from 0V to 10V. The image shows there is degraded performance towards both AVDD and ground. This is due to the linear ranges of the amplifiers in the Component Selection section. To improve performance at these extremes, the power supplies can be adjusted so that the linear range of both amplifiers fall within the full-scale range of the ADC.
The bandwidth is simulated at approximately 7MHz at the –3dB point. The bandwidth is limited by both the OPA192 device and the RC charge bucket circuit (Rfilt and Cfilt). The bandwidth of the RC circuit is shown in the following equation to be 8.2MHz. The OPA192 device has a 10-MHz bandwidth which also affects the overall bandwidth of the circuit. The bandwidth goal was selected to be two times greater than that of the sampling frequency to allow for proper settling.
The following simulation shows settling for a 9-V DC input signal. This type of simulation shows that the sample and hold kickback circuit is properly selected to within ½ of a LSB (91.5µV). See the Introduction to SAR ADC Front-End Component Selection video for detailed theory on this subject.
This section details a simplified noise calculation for a rough estimate. Noise from the OPA192 is attenuated by the resistor divider as shown:
Resistor divider noise:
OPA365 noise density:
Total noise:
Note that calculated and simulated values match well. See the Calculating the Total Noise for ADC Systems video for detailed theory on this subject.
This performance was measured on a modified version of the ADS7056EVM with a 2-kHz input sine wave. The AC performance indicates SNR = 74.4dB, and THD = –84.07dB, which matches well with the specified performance of the ADC, SNR = 74.9dB and THD = –85dB. This test was performed at room temperature. See the Introduction to Frequency Domain video for more details on this subject.
Parameter | Data Sheet Specification (Typ) | Measured Result |
---|---|---|
SNR | 74.9dB | 74.4dB |
THD | –85dB | –84.07dB |
Device | Key Features | Link | Other Possible Devices |
---|---|---|---|
ADS7056 | 14-bit resolution, SPI, 2.5-MSPS sample rate, single-ended input, AVDD/VREF input range 2.35V to 3.6V, DVDD 1.65V to 3.6V | 14-bit 2.5-MSPS ultra-low-power ultra-small-size SAR ADC with SPI | Precision ADCs |
OPA192 | 8-kHz bandwidth, rail-to-rail output, 450-nA supply current, unity gain stable | High-Voltage, Rail-to-Rail Input/Output, 5µV, 0.2µV/˚C, Precision Operational Amplifier | Operational amplifiers (op amps) |
OPA365 | 50-MHz bandwidth, rail-to-rail output, zero-crossover, low offset 100µV, low noise 4.5nV/√ Hz, slew rate 25V/µs | 2.2V, 50MHz, Low-Noise, Single-Supply Rail-to-Rail Operational Amplifier |
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