How to drive a low-power 16-bit ADC using multiple single-ended signals

匹配传感器输出和 ADC 输入范围可能很难，尤其是要面对当今传感器所产生的多种输出电压摆幅时。本文为不同变化范围的差分、单端、单极性和双极性信号提供简便但高性能的 ADC 输入驱动器解决方案，本文的所有电路採用了 LTC2383-16 ADC 单独工作或与 LT6350 ADC 驱动器一起工作来实现 92dB SNR。

　　The LTC2383-16 is a low noise, low power, 1Msps, 16-bit ADC with a ±2.5V fully differential input range. The LT6350 is a rail-to-rail input and output, low noise, low power single-ended to differential converter/ADC driver with fast settling time. Using the LT6350, single-ended input ranges of 0V to 2.5V, 0V to 5V and ±10V can be easily converted to the ±2.5V fully differential input range of the LTC2383-16.

　　Fully differential drive

　　Figure 1 shows the basic building block used for all circuits described in this article. This basic building block is used to DC couple fully differential signals to the LTC2383-16 analog inputs. Resistors R1, R2, and capacitor C1 limit the input bandwidth to approximately 500kHz. Resistors R3 and R4 mitigate the effects of ADC input sampling spikes that can interfere with the sensor or ADC driver inputs.

　　Figure 1: Fully differential drive circuit

　　This circuit is useful for sensors with low impedance differential outputs. The common mode voltage driving AIN+ and AIN– must be equal to VREF/2 to meet the common mode input range requirement of the LTC2383-16.

　　The circuit in Figure 1 can be AC ​​coupled to match the common-mode voltage of the ADC input to the sensor if necessary. Simply bias AIN+ and AIN– to VCM (VCM=VREF/2) through a 1k resistor and couple the sensor output to AIN+ and AIN– through a 10µF capacitor, as shown in Figure 2.

　　Figure 2: AC-coupled fully differential drive circuit

　　When driving a low noise, low distortion ADC like the LTC2383-16, choosing the right components is critical to maintaining high performance. All resistors used in these circuits are relatively low value. This keeps noise low and settling time short. Metal film resistors are recommended to reduce distortion caused by self-heating. NPO capacitors are used for C1 because they have a low voltage coefficient, which minimizes distortion.

　　Single-ended to differential conversion

　　Of course, not all sensor outputs are differential. Here are some ways to drive the LTC2383-16 with a single-ended signal.

　　0V to 2.5V single-ended input

　　The circuit shown in Figure 3 converts a 0V to 2.5V single-ended signal to a fully differential ±2.5V signal. This circuit also features high impedance inputs so that it can be driven directly with most sensor outputs. As shown in Figure 2, the common mode voltage at VIN can be matched to the ADC by AC coupling VIN. The common mode voltage of the second amplifier is set at the +IN2 pin of the LT6350. The 32k point FFT in Figure 4 shows the performance of the LTC2383-16 combined with the LT6350 when using the circuit shown in Figure 3. The measured 92dB SNR and -107dB THD closely match the typical data sheet specifications of the LTC2383-16. This shows that the degradation of the ADC specifications, if any, is minimal after inserting the single-ended to differential converter in the signal path.

　　Figure 3: Single-ended to differential converter

　　Figure 4: FFT of the circuit in Figure 3

　　AMPLITUDE: Amplitude

　　0V to 5V single-ended input

　　If a wider input range is required, the negative input of the LT6350 can be driven, allowing the input voltage to be attenuated by the first stage of the LT6350. The circuit shown in Figure 5 converts a 0V to 5V single-ended signal to a ±2.5V differential signal that drives the input of the LTC2383-16. The input impedance of this circuit is equal to R7. Increasing the value of R7 increases the input impedance, making it easier to drive. The trade-off is that if R7 is increased above 4.99k, the noise and distortion will increase slightly, as shown in Table 1.

　　Figure 5: 0V to 5V single-ended driver

　　Table 1: Noise and distortion of 0V to 5V driver versus input resistance

　　±10V single-ended input

　　Some sensors provide output voltages above and below ground. The circuit shown in Figure 6 converts a ±10V ground referenced single-ended signal to a ±2.5V differential signal that drives the input of the LTC2383-16. The input impedance is still set by R7. Table 2 shows the noise and distortion of the Figure 6 circuit as a function of input impedance.

　　Figure 6: ±10V single-ended driver

　　Table 2: Noise and distortion vs. input resistance with ±10V drivers

　　in conclusion

　　The LTC2383-16 is a low power, low noise, 16-bit ADC that easily interfaces with a wide variety of sensor outputs, including a wide range of unipolar, bipolar, differential and single-ended signals.