The circuit shown in Figure 1 is a high-speed FET-input, gain-5 instrumentation amplifier with a wide 35 MHz bandwidth and excellent ac common-mode rejection (CMR) performance of 55 dB at 10 MHz. This circuit is suitable for applications requiring high input impedance, fast instrumentation amplifiers, including RF, video, optical signal detection, and high-speed instrumentation. The high CMR and wide bandwidth characteristics also make it ideal for wideband differential line receivers.

Most discrete instrumentation amplifiers require expensive matched resistor networks to achieve high CMR performance; however, this circuit uses an integrated differential amplifier along with on-chip matched resistors to improve performance, reduce cost, and minimize printed circuit board (PCB) ) layout area.

The composite instrumentation amplifier circuit in Figure 1 has the following properties:

• Offset voltage: 4 mV (max)
• Input bias current: 2 pA (typ)
• Input common-mode voltage: −3.5 V to +2.2 V (max)
• Input differential voltage: ±3.5 V/G1 (maximum value), G1 represents the first-stage gain
• Output voltage swing: 0.01 V to 4.75 V (typ, 150 Ω load)
• −3 dB bandwidth: 35 MHz (typical, G = 5)
• Common-mode rejection: 55 dB (typ., 10 MHz)
• Input voltage noise: 10 nV/√Hz (typ, 100 kHz RTI)
• Harmonic distortion: −60 dBc (10 MHz, G = 5, V _OUT = 1 V pp, R _L = 1 kΩ)

Most fully integrated instrumentation amplifiers are manufactured using bipolar or complementary bipolar processes and are optimized for low-frequency applications with high CMR performance at 50 Hz or 60 Hz. However, there is a growing need for wide-bandwidth instrumentation amplifiers used in video and RF systems to amplify high-speed signals and provide common-mode rejection of high-frequency noise signals.

When very high speed, wide bandwidth instrumentation amplifiers are required, a common approach is to use two high input impedance discrete op amps to buffer and amplify the differential input signal in the first stage and then in the second stage Configure a single amplifier as a differential amplifier to provide differential to single-ended conversion. This configuration is often called a three-op-amp instrumentation amplifier. This method requires the use of 4 relatively expensive precision matched resistors to achieve good CMR performance. If there is an error in the match, there will be an error in the final output.

The circuit shown in Figure 1 can solve this problem. This circuit uses the ADA4830-1 integrated high-speed difference amplifier. Laser trimmed thin film resistors are matched with extremely high accuracy, eliminating the need for 4 relatively expensive precision matched external resistors.

In addition, using a high-speed dual-channel ADA4817-2 as the input stage amplifier allows the composite instrumentation amplifier to provide up to 80 MHz of bandwidth while maintaining a total circuit gain of 2.5.

The dual-channel ADA4817-2 amplifier and integrated ADA4830-1 difference amplifier in a single 4 mm × 4 mm LFCSP package significantly reduce board space, thereby lowering the design cost of large systems.

This circuit can be used in noisy environments because both the ADA4817-2 and ADA4830-1 provide low noise and excellent CMR performance at high frequencies.

The circuit is based on a traditional three-op-amp instrumentation amplifier topology, with two op-amps for the input gain stage and a difference amplifier for the output stage. This circuit has a gain of 5 and a bandwidth of 35 MHz

FET amplifier input gain stage

The ADA4817-2 (dual) FastFET amplifier is a unity-gain stable, ultra-high-speed voltage feedback amplifier with FET inputs. Fabricated using Analog Devices' proprietary ultra-fast complementary bipolar (XFCB) process, these amplifiers operate with extremely low noise, very high input impedance and high speed, making them suitable for applications requiring high speed and high source impedance.

The ADA4817-2 op amp is configured to share the R _G gain resistor. For differential inputs, the circuit gain is 1 + 2RF _/ R _G . With a common-mode input, no current flows through the R _G gain resistor. Therefore, this circuit acts as a buffer at common-mode input. A second stage differential amplifier then effectively removes the common-mode input.

The unity gain bandwidth product f _u of the ADA4817-2 is equal to 410 MHz. Its closed-loop bandwidth can be approximately calculated by the following formula:

f _{−3 dB} = f _U /G1

Among them, G1 is the gain of the first stage.

For this circuit, since the first stage closed-loop gain is 10, the −3 dB bandwidth is estimated to be 41 MHz. This value is very close to the test bandwidth of 35 MHz.

Parasitic capacitance and capacitive loads on the PCB board can cause the first gain stage to oscillate. This problem can be alleviated by using a low value feedback resistor and using a feedback capacitor.

This circuit uses a feedback resistor of 200 Ω. The feedback capacitor CF is 2 p _F for optimal bandwidth flatness.

Differential Amplifier and CMR

The ADA4830-1 is a high-speed differential amplifier with a wide common-mode voltage range that combines high speed and precision. It provides a fixed gain of 0.5 V/V with a −3 dB bandwidth of 84 MHz. With on-chip laser trimmed resistors, the device has a typical CMR of 55 dB at 10 MHz.

CMR is an extremely important specification parameter of the instrumentation amplifier and mainly depends on the ratio matching of the four resistors used in the second-stage differential amplifier, as shown in Figure 2.

Typically, the worst-case CMR is given by: =

where Kr is the individual resistor tolerance expressed as a decimal. The above equation represents a worst-case CMR of 34 dB, where the 4 resistors have the same nominal value (1% tolerance). This circuit uses a single-chip ADA4830-1 differential amplifier instead of discrete resistors. The amplifier integrates laser-trimmed thin film resistors on-chip, resulting in excellent CMR performance and saving PCB space. The CMR is 65 dB at DC and 55 dB at 10 MHz.

Differential and Common-Mode Voltage Considerations

To maximize the input voltage range and simplify power requirements, the circuit uses a ±5 V supply for the first stage and +5 V for the second stage. The maximum differential input range is determined by the output swing of the ADA4817-2. The ADA4817-2 output swings to ±3.5 V when operating from a ±5 V supply. Therefore, the maximum allowed differential input is ±3.5 V/G1, where G1 represents the first-stage gain. Note that there is a trade-off between the maximum allowed differential input and the first-stage closed-loop gain.

Next, analyze the common-mode voltage limitations. The common-mode voltage at the input of the ADA4817-2 must be between −VS and +VS − 1.8 V, which is −5 V to +2.2 V using a ±5 V supply. The ADA4817-2 output swing is limited to ±3.5 V when operating from a ±5 V supply (refer to the ADA4817-2 data sheet). Therefore, the output swing of the ADA4817-2 limits the circuit's negative input common-mode voltage to −3.5 V, allowing the composite circuit to have an input common-mode range of −3.5 V to +2.2 V.

To obtain high performance from this circuit, good layout, grounding, and decoupling techniques must be used. For PCB layout details, please refer to Tutorial MT-031 , Tutorial MT-101 , and the article " A Practical Guide to High-Speed ​​Printed Circuit Board Layout ." Additionally, layout guidelines are provided in the ADA4817-2 data sheet and ADA4830-1 data sheet.

Circuit performance

The four most important parameters of the composite circuit were tested: CMR, −3 dB bandwidth, input-referred noise, and harmonic distortion. The test results are shown in Figures 3 to 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 a gain of 5 with a bandwidth of 35 MHz and an output load of 100 Ω. Figure 5 shows that the input-referred noise of this composite circuit is only 10 nV/√Hz at 100 kHz, and the flatband noise at higher frequencies is 8 nV/√Hz. Figure 6 shows that at 10 MHz, the circuit has a THD of 60 dBc (V _OUT =1 V pp, R _L = 1 kΩ).