The circuit shown in Figure 1 is a high speed FET input, gain of 5 instrumentation amplifier (inamp) with a wide bandwidth (35 MHz) and excellent ac commonmode rejection, CMR, (55 dB at 10 MHz). The circuit is ideal for applications where a high input impedance, fast inamp is required, including RF, video, optical signal sensing, and high speed instrumentation. The high CMR and bandwidth also makes it ideal as a wideband differential line receiver.
Most discrete inamps require expensive matched resistor networks to achieve high CMR; however, this circuit uses an integrated difference amplifier with onchip matched resistors to improve performance, reduce cost, and minimize printed circuit board (PCB) layout area.
The composite inamp circuit shown in Figure 1 has the following performance:

Figure 1. High Speed FET Input Instrumentation Amplifier (Note: Power Supply Decoupling Not Shown)

Most fully integrated inamps are fabricated on bipolar or complementary bipolar processes and are optimized for low frequency applications with high CMR at 50 Hz or 60 Hz. However, there is a growing need for wide bandwidth inamps for video and RF systems to amplify high speed signals and provide commonmode rejection of unwanted high frequency signals.
When a very high speed, wide bandwidth inamp is needed, one common approach is to use two discrete op amps with high input impedance to buffer and amplify the differential input signal in the first stage, and then configure a single amplifier as a difference amplifier in the second stage to provide a differentialtosingleended conversion. This configuration is known generally as a 3opamp inamp. This approach requires four relatively expensive precisionmatched resistors for good CMR. Errors in matching produce errors at the final output.
The circuit shown in Figure 1 solves this problem by using the ADA48301 integrated high speed difference amplifier. The lasertrimmed thin film resistors are matched to very high precision, thereby eliminating the need for four relatively expensive precisionmatched external resistors.
In addition, the use of the high speed, dual ADA48172 as the input stage amplifier allows the composite inamp to provide a bandwidth as high as 80 MHz when the overall gain of the circuit is 2.5.
The use of the dual ADA48172 amplifiers in a single 4 mm × 4 mm LFCSP package and the integrated ADA48301 difference amplifier significantly reduces board space, thereby reducing design costs for large systems.
The circuit can be used in noisy environments because both the ADA48172 and ADA48301 offer low noise and excellent CMR performance at high frequencies.
The circuit is based on the traditional 3opamp inamp topology with two op amps for the input gain stage and a difference amplifier for the output stage. The circuit has as a gain of 5 and a bandwidth of 35 MHz.
FET Amplifier Input Gain Stage
The ADA48172 (dual) FastFET amplifiers are unitygain stable,
ultrahigh speed voltage feedback amplifiers with FET inputs. These amplifiers are fabricated on Analog Devices, Inc., proprietary
eXtra Fast Complementary Bipolar (XFCB) process, which allows the amplifiers to achieve ultralow noise as well as very high input impedances and high speed, making it ideal for applications where high speed and high source impedances are required.
The ADA48172 op amps are configured so that they share the R_{G} gain resistor. The circuit has a gain of 1 + 2R_{F}/R_{G} for the differential inputs. When the inputs are commonmode, there is no current flowing through the R_{G} gain resistor. Thus, the circuit acts as a buffer for the commonmode inputs. The commonmode inputs are then effectively removed by the second stage difference amplifier.
The ADA48172 has a unitygain bandwidth product, f_{u}, of 410 MHz. Its closelooped bandwidth can be approximated by
f_{−3 dB} = f_{U}/G1
where G1 is the gain of the first stage.
For this circuit, with a first stage closedloop gain of 10, the −3 dB bandwidth is estimated to be 41 MHz. This is very close to the tested bandwidth of 35 MHz.
Parasitic capacitance in the PCB boards and capacitive loads can cause the first gain stage to oscillate. This issue can be alleviated by using low value feedback resistors, and the use of feedback capacitance.
For this circuit, a feedback resistor of 200 Ω was chosen. The feedback capacitor, C_{F}, was 2 pF for the best bandwidth flatness.
Difference Amplifier and CMR
The ADA48301 is high speed difference amplifier with a wide commonmode voltage range. It combines high speed and precision. It offers a fixed gain of 0.5 V/V, and −3 dB bandwidth
of 84 MHz. The onchip, lasertrimmed resistors yield a typical CMR of 55 dB at 10MHz. CMR is a very important specification for inamps and depends mostly on the ratio matching of the four resistors used in the second stage difference amplifier, as is shown in Figure 2.

Figure 2. Difference Amplifier

In general, the worstcase CMR is given by
where Kr is the individual resistor tolerance in fractional form. The previous equation shows that the worstcase CMR for four resistors with the same nominal values (1% tolerance) is 34 dB. Instead of using discrete resistors, this circuit uses a monolithic ADA48301 difference amplifier with onchip, lasertrimmed thin film resistors, thereby providing excellent CMR and saving PCB space. The CMR is 65 dB at dc and 55 dB at 10 MHz.
Differential and CommonMode Voltage Considerations
To maximize the input voltage range and simplify the power supply
requirements, the first stage of the circuit operates on ±5 V, whereas
the second stage operates at +5 V. The maximum differential input
range is determined by the output swing of the ADA48172. With a
±5 V supply, the ADA48172 has an output swing of ±3.5 V.
Therefore, the maximum allowable differential input is ±3.5 V/G1,
where G1 is the gain of the first stage. Note that there is a tradeoff between the maximum allowable differential input and the closedloop gain of the first stage.
The next step is to analyze the commonmode voltage restrictions. The commonmode voltage at the input to the ADA48172 must fall between −V_{S} to +V_{S} − 1.8 V, or −5 V to +2.2 V for ±5 V supplies. The output swing of the ADA48172 is limited to ±3.5 V when operating on ±5 V supplies (refer to the ADA48172 data sheet). The negative input commonmode voltage of the circuit is therefore limited to −3.5 V by the output swing of the ADA48172. Therefore, the allowable input commonmode range for the composite circuit is −3.5 V to +2.2 V.
To achieve high performance from this circuit, excellent layout, grounding, and decoupling techniques must be applied. See MT031 Tutorial, MT101 Tutorial, and the A Practical Guide to HighSpeed PrintedCircuitBoard Layout article for more detailed information regarding PCB layout. In addition, there are layout guidelines within the ADA48172 datasheet and the ADA48301 data sheet
Circuit Performance
The four most important parameters of this composite circuit,
CMR, −3 dB bandwidth, input referred noise, and harmonic distortion, are tested, and the results are shown in Figure 3 to Figure 6.
Figure 3 shows that the CMR of the composite circuit is −65 dB at dc and −55 dB at 10 MHz. Figure 4 shows that the bandwidth is 35 MHz at a gain of 5 and an output load of 100 Ω. Figure 5 shows that the composite circuit only has 10 nV/√Hz inputreferred noise at 100 kHz and a flatband noise of 8 nV/√Hz at higher frequencies. Figure 6 shows that the circuit has a THD of 60 dBc at10 MHz, with V_{OUT} =1 V _{pp} and R_{L} = 1 kΩ.

Figure 3. CMR of the CN0273


Figure 4. Frequency Response of Composite Circuit, V_{OUT} = 1 V _{pp}, R_{L} = 100 Ω


Figure 5. Input Referred Voltage Noise of Composite Circuit


Figure 6. Second (HD2) and Third (HD3) Harmonic Distortion, V_{OUT} = 1 V _{pp}, R_{L} = 1 kΩ

The overall gain of this circuit can be easily configured by the value of the gain resistor, R_{G}, shown in Figure 1. Note that with a larger overall gain, the bandwidth of this circuit decreases.
The difference amplifier at the second stage can be replaced by the AD8274 in lower speed applications. The AD8274 difference amplifier offers a fixed gain of 2. Therefore, a larger overall gain can be achieved.
To increase the input commonmode and differential range, a railtorail high speed FET input amplifier, such as the AD8065/ AD8066 that operates on ±12 V supplies and has a unitygain bandwidth of 145 MHz, can be used.
The circuit can be easily evaluated using a signal generator and an oscilloscope. The board is tested with traditional amplifier test methods using a network analyzer. For complete schematics and PCB layout, refer to the CN0273Design Support package. A photo of the board is shown in Figure 7.
Note that the CMRR data in Figure 3 was taken for a differential input voltage of 0 V. The bandwidth data in Figure 4 and the distortion data in Figure 6 were taken using a balanced differential drive source with a commonmode voltage of 0 V.

Figure 7. Photo of the EVALCN0273EB1Z Evaluation Board
