Single-Supply DC-Coupled 16-Bit, 125 MSPS Analog Front End for Bipolar Input
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.
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 (DiffAmpCalc 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 design program 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 minimizes high frequency peaking and isolates the amplifier output from the capacitance of the filter. In the Figure 1 circuit, these values are 25 Ω.
The three-pole Butterworth low-pass filter helps to roll-off second-order 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.
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.
One solution to the problem is to add a negative supply, but 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.
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 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 also shows this clipping (see Figure 3).
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.
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.
Table 1. Summary of ENOB, SINAD, and SNR Results
| Figure of Merit
(See Figure 4)
| No RCM Resistors
(See Figure 5)
| RCM Resistors
(See Figure 6)
| SINAD (dBc)
| SNR (dB)
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.
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.
The following equipment is required:
- A PC with a USB port and Windows® XP or Windows Vista® (32-bit), or Windows 7 (32-bit)
- The EVAL-FDA-1CPZ-16 evaluation board
- The AD9265-125EBZ evaluation board
- The HSC-ADC-EVALCZ FPGA-based data capture kit
- VisualAnalog software
- Analog Devices DiffAmpCalc tool
- A 3.3 V at 100 mA power supply
- A 0.9 V at 100 mA power supply
- A 6 V at 2 A wall mounted power supply (two of each)
- A 125.127 MHz Wenzel crystal oscillator (Part #500-25341)
- A 70 MHz band-pass filter
- A 125 MHz band-pass filter
- A RF source: Rohde & Schwarz SMA100A signal generator
- Coaxial cables with BNC and SMA connectors
The VisualAnalog software for the AD9265 is located at VisualAnalog and the FPGA data capture kit user guides are available at High Speed Data Capture Boards. The software is compatible with Windows XP (SP2), Windows Vista, and Windows 7 (32 bit or 64 bit). Download the VisualAnalog software and install it.
Install the evaluation software before connecting the FPGA-based data capture kit to the USB port of the PC to ensure the evaluation system is correctly recognized when connected to the PC.
Setup and Test
See the UG-074 User Guide for complete setup details on using the software and running the tests. A functional block diagram of the test setup is shown in Figure 8.
The following minor hardware changes are required on the AD9265 evaluation board to test the circuit shown in Figure 1:
- Install a SMA input connector at J2, INPUT−.
- Remove baluns from T3 and T6.
- Remove capacitors from C2 to C4, C15, C96, and C71.
- Remove resistors from R1, R15, R16, R22, R23, and R47.
- Install 0 Ω in R1, R22, R23, R32, C3, C25, C71, and C96.
- Install 4.7 pF capacitors at R37 and R47.
- Install a 150 nH inductor across Pin 1 and Pin 6 of the T6 footprint.
- Install a 150 nH inductor across Pin 3 and Pin 4 of the T6 footprint.
- Install a 10 pF capacitor across Pin 1 and Pin 3 of the T6 footprint.
- Remove the P18 jumper.
|AD9265||16-Bit, 125 MSPS/105 MSPS/80 MSPS, 1.8 V Analog-to-Digital Converter|
Ultralow Noise Drivers for Low Voltage ADCs
|ADA4930-2||Ultralow Noise Drivers for Low Voltage ADCs||