Circuit functions and advantages

Many applications require high-resolution, differential input ADC to convert single ended analog signals, whether bipolar or unipolar. The DC coupling circuit can convert the single ended input signal into differential signal, which is suitable for driving the 18 bit, 1 MSPs device ad7982 in pulsar series ADC. The circuit adopts single ended to differential driver ada4941-1 and ultra-low noise 5.0 V reference voltage source adr435. It can accept many types of single ended input signals, including bipolar or unipolar signals in the range from high voltage to low voltage.

The whole circuit remains directly coupled. If you need to focus on the circuit board space, you can use small package products. All ICs shown in Figure 1 can provide 3 mm & tips; 3 mm lfcsp or 3 mm & ties; 5 mm MSOP small package.

  Figure 1: single ended to differential DC coupled driver circuit (schematic diagram)

Figure 1: single ended to differential DC coupled driver circuit (schematic diagram)

Circuit description

The differential input voltage range of the ad7982 is set by the voltage on the ref pin. When VREF = 5 V, the differential input voltage range is ± VREF = ± 5 v. The voltage gain (or attenuation) from the single ended source VIN to the output of ada4941-1 is set by the ratio of R2 to R1. The ratio of R2 to R1 shall be equal to the ratio of VREF to input voltage peak to peak Vin. When the peak to peak value of single ended input voltage is 10 V and VREF = 5 V, the ratio of R2 to R1 shall be 0.5. The signal on OUTN is the inverse of the output signal. The absolute value of R1 determines the input impedance of the circuit. The feedback capacitance CF is selected according to the required signal bandwidth, which is about 1 / (2 π r2cf). 20 Ω resistor and 2.7 NF capacitor form a 3 MHz single pole low-pass noise filter. Resistors R3 and R4 set the in of ad7982? Common mode voltage at the input.

This common mode voltage value is equal to voffset2 & times; (1 + R2 / R1), where voffset2 = VREF & names; R3/(R3 + R4)。 Resistors R5 and R6 set the common mode voltage at the in + input of the ADC. This voltage is equal to voffset1 = VREF × R5/(R5 + R6)。 The common mode voltage of ADC (equal to voffset1) should be close to VREF / 2, which means R5 = R6. Table 1 lists some standard 1% allowable resistance values suitable for common input voltage ranges.

Table 1: circuit values and voltages suitable for common input voltage ranges

  Table 1: circuit values and voltages suitable for common input voltage ranges

Please note that ada4941-1 uses + 7 V and? 2 V power supply. Since the swing of each output must reach 0 V to + 5 V, the positive power supply voltage should be hundreds of MV higher than + 5 V and the negative power supply voltage should be hundreds of MV lower than 0 V. This circuit selects + 7 V and? 2 V supply voltage+ The 7 V power supply can also provide sufficient margin to supply power to adr435. Other voltages can also be used as long as the absolute maximum total supply voltage on ada4941-1 does not exceed 12 V and meets the margin requirements of adr435.

The ad7982 requires a + 2.5 V supply for VDD power supply and a vio power supply (not shown in Figure 1), which can be between 1.8 V and 5 V, depending on the I / O logic interface level.

This circuit is not sensitive to power timing. Under transient overvoltage conditions, the ad7982 input can withstand a current of up to ± 130 ma.

The ad7982 SPI compatible serial interface (not shown in Figure 1) can daisy chain several ADCs to a single three wire bus using SDI input, and provide an optional free busy indication. When using independent power vio, the device is compatible with 1.8V, 2.5V, 3V and 5V logic.

In order to achieve the ideal performance of the circuit discussed in this paper, excellent wiring, grounding and decoupling technologies must be used. At least four layers of PCB shall be used: one grounding layer, one power layer and two signal layers.

All IC power pins must be 0.01 μ F to 0.1 μ F low inductance, multilayer ceramic capacitors (MLCC) are decoupled from the ground (not shown in Figure 1 for simplicity) and should follow the recommendations in the data manuals of the IC referenced in the “learn more” section.

For the recommended wiring method and key device location, refer to the product * evaluation board. Please check it on the product home page of the device (see the “learn more” section).

Common changes

Adr43x series reference voltage source can provide various reference voltage values connected with ADC.

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