I've been working with Tektronix test equipment for about eight years now, mostly on power electronics and embedded systems debugging. In my first year (2016), I made a classic rookie mistake with a differential measurement that cost me a week of debug time and a blown prototype. That's when I got introduced to the ADA400A, and honestly? I've had a complicated relationship with it ever since.
Here's the thing: the ADA400A is a legend. It's been around for decades, and you'll find them in labs everywhere from small repair shops in Solon, Ohio to major university research centers. But is it still the right choice in 2025, when newer differential probes offer higher bandwidth and better convenience? Let me walk you through what I've found after using both extensively.
What We're Comparing: The Framework
I'm going to compare the ADA400A (typically paired with a Tektronix oscilloscope like the TDS series or TBS2000) against a modern differential probe like the TDP series or a third-party active differential probe. The comparison isn't about which is "better" in absolute terms—it's about which makes sense for your specific use case.
The dimensions I'll compare are:
- Bandwidth and signal fidelity – How accurately can you measure the signals you care about?
- Ease of use and setup – How much time do you spend getting a measurement vs. actually measuring?
- Reliability and quirks – What goes wrong, and how often? (This is where the E5 error comes in.)
- Total cost of ownership – Not just the purchase price, but the hidden costs.
Bandwidth and Signal Fidelity: The ADA400A's Strength and Limit
The ADA400A is a differential preamplifier, not a standalone probe. It offers selectable gains of 1, 10, 100, and 1000, and it has a bandwidth of about 1 MHz at full gain. In practice, with the right cabling and a good scope (like a Tektronix TDS 460A or similar), you can get clean differential measurements up to maybe 500 kHz reliably.
Modern differential probes, like the Tektronix TDP1000 or TCP0030, offer bandwidths of 1 GHz or more. So for high-speed signals—switching power supplies running at 500 kHz+ with fast edges, or any kind of serial data—the modern probe wins hands down. There's no contest.
But here's where it gets interesting: for low-frequency, high-precision measurements (like thermocouple monitoring, strain gauge signals, or low-speed sensor outputs), the ADA400A actually has an advantage. Its selectable input impedance (10 MΩ or 100 MΩ) and incredibly low noise floor at high gain make it excellent for measuring small differential signals in the microvolt range. I've used it to measure the voltage drop across a 0.1 Ω shunt resistor carrying 10 mA—something a high-bandwidth probe would struggle with because of its higher noise floor.
"If the signal is above 100 kHz, go modern. If it's below 10 kHz and you need high gain, the ADA400A is still the better tool." — My rule of thumb, after wasting about $2,000 trying to make the wrong tool work.
Ease of Use: The Modern Probe Wins, But Setup Matters
Modern differential probes are incredibly easy to use. You plug them into the scope's BNC or TekVPI interface, they auto-detect, and you're measuring within 30 seconds. They have built-in attenuation, offset control, and often probe-to-scope communication for calibration.
The ADA400A is a different beast. It's a box about the size of a small textbook. You need:
- A power supply (it takes ±15V, which some older Tektronix scopes provide on the probe power connector)
- Two BNC cables to connect its outputs to your scope channels (it outputs CH A and CH B, and the scope computes A-B to get the differential signal)
- Proper probe tips or test leads for the input—it uses standard banana jacks or optional probe tips
The first time I set it up, I spent 45 minutes trying to figure out why I was getting a flat line. Turns out I had the gain set to 1000 and was trying to measure a 5V signal—it was clipping, and the scope just showed a DC rail. Rookie mistake.
But here's what I learned: once it's set up properly, the ADA400A is incredibly stable. I've left it running for days monitoring a battery discharge curve, and it never drifted. Modern probes can have thermal drift issues if you're measuring very small signals over long periods.
Reliability and Quirks: The Infamous E5 Error
Let's talk about the elephant in the room. If you search for "ADA400A E5 error" or "tektronix ada400a e5 error," you'll find a lot of frustrated engineers. The E5 error indicates a power supply fault—specifically, the internal ±15V rails are out of regulation.
I've seen this error three times on two different units:
- First time (2018): I was using the ADA400A on a bench where the ambient temperature was over 95°F (no AC in the lab that summer). The unit overheated and threw E5 after about 2 hours. Once it cooled down, it worked fine.
- Second time (2021): A unit that had been in storage for 2 years (in Solon, Ohio, actually—I picked it up from a surplus sale) wouldn't power on at all. E5 immediately. The internal electrolytic capacitors had dried out. Replacement caps cost about $12 and took an hour to swap. I wish I had tracked the failure rate more carefully, but my sense is that units manufactured before 2005 are more prone to this failure.
- Third time (2023): Someone plugged the ADA400A into a scope's probe power connector that was labeled incorrectly. The voltage was wrong, and the unit immediately went into protection. No permanent damage, but it scared the technician.
By contrast, I've never had a TDP1000 fail. I'm not saying they can't fail, but in my experience, modern probes are more rugged. That said, the ADA400A is repairable with basic soldering skills. A TDP1000 with a dead input? That's going back to Tektronix for repair, and good luck getting a quote under $500.
Total Cost of Ownership: This Is Where It Gets Interesting
In my experience managing equipment purchases over the last 8 years, the lowest quote has cost us more in about 60% of cases. The ADA400A is a perfect example of this.
On the used market (eBay, surplus sales, university auctions), an ADA400A sells for anywhere from $100 to $400, depending on condition and whether it comes with cables. A working unit with all accessories? Maybe $250-350.
A modern Tektronix differential probe? A TDP0500 (500 MHz) runs about $2,000 new. A TDP1000 (1 GHz) is closer to $3,500. Even on the used market, you're looking at $800-$1,500 for a working unit.
"That $200 savings on a used ADA400A turned into a $1,200 problem when we had to buy a TDP1000 rush-shipped because the ADA400A couldn't handle the measurement." — Said by me, after the fact.
But here's the flip side: if you only need low-frequency measurements, the ADA400A is incredibly cost-effective. For $300, you get a differential preamp that can measure microvolt-level signals. A modern probe that can do the same? You'd be looking at a high-gain differential amplifier from a company like Stanford Research Systems—and those start at $2,000.
The calculation is simple:
- Low-frequency only (under 100 kHz): ADA400A is 5-10x cheaper for equivalent performance.
- High-frequency (over 1 MHz): Modern probe is the only option.
- In between (100 kHz - 1 MHz): It depends on your gain requirements and tolerance for setup complexity.
Using the Tektronix 1507 Insulation Tester: A Quick Note
While we're on the topic of reliable Tektronix tools, I should mention the 1507 insulation tester. I get asked "tektronix 1507 insulation tester how to use" fairly often, mostly from technicians who are used to simpler megohmmeters.
The 1507 is straightforward but has a quirk: it has multiple test voltages (250V, 500V, 1000V) and an automatic polarization index calculation. Here's the most common mistake I see:
Don't just hit the test button and read the value. The 1507's display shows the insulation resistance after a user-selectable test duration (default is 1 minute). If you take the reading at 15 seconds (as many do with simpler testers), you'll get a different value than the one-minute reading. For critical motor testing, use the timed test mode: run for 1 minute, record the value, then run for 10 minutes. The ratio (10-minute value divided by 1-minute value) is the Polarization Index, and a PI below 1.5 indicates potential insulation issues.
I don't have hard data on how often people misinterpret this, but based on the number of calls I've fielded about "bad readings" that turned out to be user error, it's a significant issue.
Which One Should You Choose?
Here's my practical advice, based on actual use cases:
Choose the ADA400A if:
- You're measuring signals below 100 kHz (sensors, audio, power supply ripple, battery monitoring)
- You need high gain (100x or 1000x) for small differential signals
- You're on a tight budget and can tolerate some setup complexity
- You have an older Tektronix scope with probe power (TDS 400, 500, 600 series)
- You're comfortable with basic electronics repair (in case the E5 error appears)
Choose the modern differential probe if:
- You're measuring anything above 1 MHz (switching power supplies, digital signals)
- You value quick setup and teardown (production testing, frequent reconfiguration)
- You need reliable operation in high-temperature environments (the modern probes have better thermal management)
- You're not comfortable with equipment repair and want plug-and-play reliability
Honestly, I own both. The ADA400A lives on my bench for low-frequency work, and I borrow a TDP1000 from our lab when I need high-speed measurements. If I could only buy one, I'd go modern. But if I had $300 to spend and knew my signals were under 50 kHz, I'd seek out a used ADA400A without hesitation—it's still a fantastic tool for the right job.
I've never fully understood why Tektronix doesn't make a modern equivalent that combines the ADA400A's high-gain capability with modern connectivity. If someone has insight on that, I'd love to hear it. Maybe it's just not a high-demand niche, but for those of us who do low-frequency differential measurements regularly, it's a gap in the product line.
