AN-2539: Single-Supply, DC-Coupled 16-Bit, 125 MSPS Analog Front End for Bipolar Input Signals
Circuit Function And Benefits
The circuit shown in Figure 1 solves a problem often encountered in dc-coupled, single-supply systems when interfacing bipolar input signals to differential input, low voltage analog-to-digital converters (ADCs). The technique ensures the proper common-mode voltage level at the input to the differential drive amplifier by controlling the input common-mode level using two level shifting resistors. The output common-mode voltage is established independently by applying the correct voltage to the VOCM pin of the ADA4930-1 differential driver.
Figure 1. High Speed, Single-Ended to Differential ADC Driver (Simplified Schematic: All Connections and Decoupling Not Shown).
This flexible arrangement allows the ADA4930-1 differential driver to operate on a single 3.3 V supply, while the AD9265 16-bit 125 MSPS ADC operates on a 1.8 V supply, thereby minimizing the total circuit power dissipation.
In broadband applications, the frequency range of interest often includes dc. To maximize the dynamic range of a differential input ADC, typical input signals can be relatively large, thereby requiring the differential driver to operate at lower gain settings. Under these conditions, the input common-mode voltage of the differential driver must stay within the specified range.
Independent control of the input and output common-mode voltage of differential amplifiers is often required in direct coupled single-supply applications such as processing demodulator outputs with high input common-mode voltages, x-ray applications with a dc component added to a differential component, and other places where the differential driver must handle low value input common-mode voltages. Low input common-mode voltage applications can include either single-ended or differential inputs, with zero, bipolar, or negative inputs.
Circuit Description
Modern high speed ADCs are normally driven by differential amplifiers for best performance. Typical differential drivers give best ac performance when operating at gains of two or less, and in single-supply applications, full-scale input signals frequently exceed the input common-mode voltage range of the ADC driver.
To avoid common-mode voltage problems with differential amplifiers, the circuit must be carefully analyzed. The design equations and analysis for the ADA4930-1 differential driver are contained in its data sheet; however, the Analog Devices, Inc., Differential Amplifier Calculator™ design tool allows complete analysis of the circuit by applying nodal analysis and presents the results in graphical format.
The circuit in Figure 1 uses the ADA4930-1 because of its ability to operate on a 3.3 V single supply with an output common-mode voltage (VOCM) of 0.9 V, the common-mode level best suited for 1.8 V ADCs, such as the AD9265.
To optimize noise performance and minimize its negative impact on the signal-to-noise-and-distortion ratio (SINAD), an RFx value of 249 Ω was selected. The values for the RGx and RTx were then determined using the DiffAmpCalc design tool set for a gain of 0.511 measured from VIN to the differential output voltage (VOD).
The input signal of Figure 1 comes from a 50 Ω RF source and drives the band-pass filter. To keep the differential amplifier source impedances balanced, an ac coupling capacitor of 0.1 µF in series with a 49.9 Ω resistor is connected to the unused input, as shown in Figure 1. The impedance of this capacitor is low enough to act as an ac short circuit for a 70 MHz center frequency signal.
The input common-mode range for the ADA4930-1 operating on a single 3.3 V supply is 0.3 V to 1.2 V. The two input common-mode resistors, RCM1 and RCM2, connect between the differential amplifier input pins and the reference voltages, VREF1 and VREF2, ensure that the input common-mode voltage does not go below 0.3 V for full-scale bipolar input signals.
Without the common-mode bias resistors, the input common-mode voltage of the ADA4930-1 goes below 0.3 V, and clipping occurs for a full-scale signal.
For convenience, VREF1 and VREF2 are each connected to VCC, the 3.3 V single supply. The connection to 3.3 V raises the nominal input common-mode voltage to accommodate the negative input signal swing. The technique for calculating the values for the common-mode resistors is described in the ADA4930-1 data sheet.
It is common practice to include small valued snubber resistors in series with the outputs of the differential amplifier. This practice minimizes high frequency peaking and isolates the amplifier output from the capacitance of the filter. In the Figure 1 circuit, these resistor values are 25 Ω.
The three-pole Butterworth low-pass filter helps to roll off secondorder and third-order harmonics and reduces noise at the ADC input. An odd-order filter was selected so that the final filter capacitor is in parallel with the input capacitance of the AD9265.
The Butterworth filter was designed for a cutoff frequency of 100 MHz, an input impedance of 50 Ω, and an output impedance of 1 kΩ. Filter components were rounded to standard values and further optimized for best system performance.
The 10 kΩ resistor in parallel with the ADC input was chosen to be as large as possible to minimize attenuation drops in the signal path. The proximity of the ADA4930-1 to the AD9265 minimizes transmission line effects at 70 MHz. Therefore, traditional double termination methods between the driver output and the ADC input were not implemented.
When driving the AD9265, care was taken to not overdrive the input of the ADC. The maximum output of the ADA4930-1 operating on a 3.3 V supply is 1.74 V, which is within the maximum input voltage specification of the AD9265.
Common-mode Voltage Analysis
The basic starting point of the design is shown in Figure 2 after inputting the appropriate values into the DiffAmpCalc tool. Note that the input signal is 1.4 V p-p, resulting in signals at the +IN and −IN inputs that go as low as 0.305 V. Larger signals cause clipping, as is shown in Figure 3.
Figure 2. DiffAmpCalc Design Analysis for Low Level Input Signal, Single 3.3 V Supply, VOCM = 0.9 V.
Figure 3. DiffAmpCalc Design Analysis for Full-Scale Input Signal, 3.3 V Supply, VOCM = 0.9 V, Showing the Effects of Clipping.
One solution to the problem is to add a negative supply; however, the maximum supply voltage of 5.5 V cannot be exceeded, which prevents the use of ±3.3 V supplies. Although a +3.3 V, −1 V dual supply system works, this is inconvenient and adds power.
The addition of the two RCMx resistors, as shown in Figure 1, is the ideal solution and raises the nominal common-mode voltage on the ADA4930-1 from 0.489 V to 0.860 V using 887 Ω resistors. The maximum negative and positive swings at the +IN and −IN inputs are now 0.61 V and 1.11 V, respectively, which are within the allowable range of 0.3 V to 1.2 V.
Circuit Performance
Figure 4 shows the performance of the AD9265 evaluation board directly coupled to an external band-pass filter with a 70 MHz center frequency and a sampling rate of 125 MSPS. The standard AD9265 evaluation board configuration converts the signal from single-ended to differential with an RF balun.
Figure 4. VisualAnalog FFT of the AD9265 Driven with Balun.
Figure 5 shows the Figure 1 single-supply design using the AD9265 and the ADA4930-1 without the 887 Ω bias resistors. The effects of clipping are evident. The DiffAmpCalc analysis also shows this clipping (see Figure 3).
Figure 5. VisualAnalog FFT of the ADA4930-1 and AD9265 with RCM1 and RCM2 Removed Shows Effects of Clipping.
Figure 6 shows the performance of the ADA4930-1 operating on a 3.3 V single supply, with the input common-mode resistors, RCM1 and RCM2, connected. Additionally, the balun and the RC filter on the AD9265 evaluation board were removed and replaced with a three-pole Butterworth filter, as shown in Figure 1.
Figure 6. VisualAnalog FFT of the ADA4930-1 and AD9265 with RCM1 and RCM2 as shown in Figure 1.
Using effective number of bits (ENOB), SINAD, and signal-to-noise ratio (SNR) as figure of merits, Table 1 compares the results from Figure 4, Figure 5, and Figure 6.
| Figure of Merit | Baseline (See Figure 4) | No RCM Resistors (See Figure 5) | RCM Resistors (See Figure 6) |
| ENOB | 12.4 | 3.8 | 12.1 |
| SINAD (dBc) | 76.7 | 24.1 | 75.1 |
| SNR (dB) | 76.9 | 24.2 | 75.5 |
The inclusion of input common-mode resistors with the primary function of independently shifting the input common-mode level minimally affects performance, as shown in Table 1. For example, ENOB is 12.4 before the inclusion of the RCM resistors and 12.1 with the RCM resistors. The slight reduction of ENOB can be attributed to the small increase of the noise floor due to the 4.7 nV/√Hz output noise density of the ADA4930-1 in the configuration shown in Figure 1. This value was calculated using the DiffAmpCalc tool. Therefore, the input and output common-mode levels of an ADC driver can be controlled independently with the addition of two resistors, RCM1 and RCM2, while maintaining excellent ENOB, SINAD, and SNR performance.
Common Variations
Changing the feedback and gain resistors of the ADA4930-1 is one variation to the circuit shown in Figure 1. Increasing the feedback and gain resistors to 499 Ω marginally increases the noise floor such that performance is slightly impacted (see Figure 7).
Figure 7. VisualAnalog FFT of ADA4930-1 and AD9265 Using Feedback and Gain Resistors of 499 Ω.
Although the impact of changing the gain and feedback resistors is not significant, ENOB, for example, decreases from 12.1 bits to 11.9 bits.
Another variation to Figure 1 includes using alternate ADCs, such as the AD9255 (14-bit, 125 MSPS), AD9258 (dual 14-bit, 125 MSPS), or AD9268 (dual 16-bit, 125 MSPS).
For applications requiring a dual driver, such as I/Q receivers based on the dual AD9258 or AD9268, the ADA4930-2 driver is available.