The circuit shown in Figure 1 solves problems often encountered when interfacing bipolar input signals with differential input, low-voltage analog-to-digital converters (ADCs) in DC-coupled single-supply systems. This technique uses two level-shifting resistors to ensure the correct common-mode level at the input of the differential driver amplifier by controlling the input common-mode level. The output common-mode voltage is achieved independently by applying the correct voltage to the V _{OCM pin of the} ADA4930-1 differential driver .

This flexible approach allows the ADA4930-1 differential driver to operate from a single 3.3V supply while the 16-bit, 125MSPSADC AD9265 operates from a 1.8V supply, thereby minimizing total circuit power consumption.

In broadband applications, the target frequency range usually includes DC. To maximize the dynamic range of a differential input ADC, the typical input signal can be increased appropriately, which requires the differential driver to operate at a lower gain setting. Once these conditions are met, the input common-mode voltage of the differential driver must also remain within the specified range.

In direct-coupled single-supply applications, independent control of the input and output common-mode voltages of a differential amplifier is often required; these applications include: handling demodulator outputs with high input common-mode voltages, X-ray applications where DC devices are connected to differential devices , and those applications where the differential driver must handle low input common-mode voltages. Low input common-mode voltage applications may include single-ended or differential inputs, and the inputs may be zero inputs, bipolar inputs, or negative inputs.

Modern high-speed ADCs are often driven by differential amplifiers for optimal performance. Typical differential drivers achieve optimal ac performance at gains of 2 or less, and in single-supply applications, the full-scale input signal frequency exceeds the input common-mode voltage range of the ADC driver.

To avoid common-mode voltage problems when using differential amplifiers, the circuit must be carefully analyzed. The design formulas and analysis for the ADA4930-1 differential driver can be found in its data sheet; and Analog Devices' differential amplifier calculator ( DiffAmpCalc design tool ) allows a complete analysis of the circuit in the form of node analysis and the results in a graphical format express.

_{The circuit in Figure 1 uses the ADA4930-1 because it can output a common-mode voltage (V OCM} ) of 0.9 V from a single 3.3 V supply , a common-mode level that is best suited for a 1.8 V ADC such as the AD9265.

To optimize noise performance and minimize its negative impact on signal-to-noise ratio (SINAD), an R _Fx value of 249Ω was chosen. Then, using the DiffAmpCalc software design tool, the gain from V _IN to the differential output voltage (VOD) was measured to be 0.511 to determine the R _Gx and R _Tx values.

The input signal in Figure 1 is derived from 50Ω RF and drives a bandpass filter. To keep the differential amplifier source impedance balanced, connect a 0.1µF AC coupling capacitor in series with a 49.9Ω resistor to the unused input, as shown in Figure 1. The impedance of this capacitor is low enough to serve as an AC short circuit signal centered at 70 MHz.

Operates from a single 3.3 V supply and uses an input common-mode voltage range of 0.3 V to 1.2 V for the ADA4930-1. Two input common-mode resistors R _CM1 and R _CM2 are connected to the differential amplifier input pin and the reference voltages V _REF1 and V _REF2 to ensure that the input common-mode voltage is not less than 0.3 V for a full-scale bipolar input signal.

Without the common-mode bias resistor, the input common-mode voltage of the ADA4930-1 is less than 0.3 V, and clipping will occur with full-scale signals.

For convenience, V _REF1 and V _REF2 are connected to a single 3.3 V supply respectively. The connection of V _CC to the 3.3 V supply increases the nominal input common-mode voltage to accommodate the input signal swing. Tips for calculating common-mode resistance can be found in the ADA4930-1 data sheet.

It is very common to use a small value buffer resistor in series with the output of a differential amplifier. Doing this minimizes high frequency peaks and isolates the amplifier output from the filter capacitance. In the circuit shown in Figure 1, these values ​​are 25Ω.

The 3-pole Butterworth low-pass filter helps roll off second- and third-order harmonics and reduces ADC input noise. Choose an odd-order filter so that the final filter capacitor is in parallel with the input capacitance of the AD9265.

Butterworth filters are designed for a cutoff frequency of 100MHz, an input impedance of 50Ω, and an output impedance of 1Ωk. Filter component values ​​are rounded to standard values ​​and further optimized for optimal system performance.

Choose a 10Ωk resistor in parallel with the ADC input, with the largest value possible to minimize attenuation in the signal path. The close proximity of the ADA4930-1 and AD9265 minimizes transmission line effects at 70 MHz. Therefore, traditional termination between the driver output and ADC input is not used.

When driving the AD9265, care should be taken not to overdrive the ADC input. The maximum output of the ADA4930-1 from a 3.3 V supply is 1.74 V, which is within the maximum input voltage specification of the AD9265.

Common Mode Voltage Analysis

Figure 2 shows the basic entry point for the design after entering the appropriate values ​​into the DiffAmpCalc tool. Note that the input signal is 1.4 V pp, so the +IN and −IN inputs have signals as low as 0.305 V. Larger signals can cause clipping, as shown in Figure 3.

One way to solve the problem is to add a negative supply, but since the 5.5 V maximum supply voltage cannot be exceeded, a ±3.3 V supply cannot be used. Although it is possible to use a +3.3 V, −1 V dual supply system, this is inconvenient and increases power consumption.

As shown in Figure 1, the addition of two RR _CMx resistors is the ideal solution and increases the nominal common-mode voltage on the ADA4930-1 from 0.489 V to 0.860 V through the 887Ω resistor. The maximum negative and positive swings of the +IN and −IN inputs are now 0.61V and 1.11V respectively, within the allowed range of 0.3V to 1.2V.

Circuit performance

Figure 4 shows the performance of the AD9265 evaluation board when coupled directly to an external bandpass filter, centered at 70 MHz and sampled at 125 MSPS. The AD9265 evaluation board comes standard with an RF balun to convert single-ended signals to differential signals.

Figure 5 shows the single-supply design of Figure 1 using the AD9265 and ADA4930-1 without the 887Ω bias resistor. The clipping effect is obvious. DiffAmpCalc also shows the impact of this clipping (see Figure 3).

Figure 6 shows the performance of the ADA4930-1 operating from a single 3.3 V supply with common-mode resistors R _CM1 and R _CM2 connected . Additionally, the balun and RC filter on the AD9265 evaluation board were removed and a 3-pole Butterfly filter instead, as shown in Figure 1.

Using effective number of bits (ENOB), SINAD, and signal-to-noise ratio (SNR) as figures of merit, Table 1 compares the results of Figure 4, Figure 5, and Figure 6.

The main function of the input common-mode resistor is to independently convert the input common-mode voltage. Adding this resistor will have almost no impact on performance, as shown in Table 1. For example, ENOB before adding the RCM resistor was 12.4, and after adding it it was 12.1. Based on the configuration in Figure 1, since the ADA4930-1 output noise density is 4.7 nV/√Hz, the slight decrease in ENOB can be attributed to a slight increase in the noise floor. This value is calculated using the DiAmpCalc tool. Therefore, by adding two resistors, R _CM1 and R _CM2 , the input and output common-mode levels of the ADC driver can be independently controlled while maintaining excellent ENOB, SINAD, and SNR performance.